Research and analysis

HAIRS risk assessment: tick-borne encephalitis

Updated 5 April 2023

About the Human Animal Infections and Risk Surveillance group

This information was prepared by the UK Health Security Agency (UKHSA) on behalf of the joint Human Animal Infections and Risk Surveillance (HAIRS) group.

HAIRS is a multi-agency cross-government horizon scanning and risk assessment group, which acts as a forum to identify and discuss infections with potential for interspecies transfer (particularly zoonotic infections).

Members include representatives from:

• UKHSA
• the Department for the Environment, Food and Rural Affairs (Defra)
• the Department of Health and Social Care (DHSC)
• the Animal and Plant Health Agency (APHA)
• the Food Standards Agency (FSA)
• Public Health Wales
• Welsh Government
• Public Health Scotland
• Scottish Government
• Public Health Agency of Northern Ireland
• the Department of Agriculture, Environment and Rural Affairs for Northern Ireland
• the Department of Agriculture, Food and the Marine
• Health Service Executive, Republic of Ireland
• Infrastructure, Housing and Environment, Government of Jersey
• Isle of Man Government
• States Veterinary Officer, Bailiwick of Guernsey

Information on the risk assessment processes used by the HAIRS group can be found on GOV.UK.

Summary

Tick-borne encephalitis (TBE) is a viral infection that can cause a meningitis-like illness in humans. The TBE virus (TBEV) can also cause disease in animals.

Prior to 2019, TBEV had not been found in the UK. Since then, TBEV infected ticks have been identified in defined areas of:

• Thetford Forest in the East of England
• the Hampshire and Dorset border
• the New Forest
• the North Yorkshire Moors

In July 2019, a European visitor became ill after being bitten by a tick in the New Forest, Hampshire, and was subsequently diagnosed as a highly probable case of TBE. In July 2020, a second probable case of TBE infection was diagnosed in a patient from Hampshire.

A third case was reported in England in September 2022, who is likely to have acquired infection in Scotland in June 2022. A fourth case was reported in England in October 2022, with a likely exposure while visiting the North Yorkshire Moors.

The third and fourth cases both tested positive for TBEV by polymerase chain reaction (PCR) testing.

Assessment of the risk of infection to the human population in the UK

Probability

Very low for the general population.

Low for high risk groups (such as those living, working or visiting affected areas, as determined by duration of time spent outside).

Impact

Low.

Level of confidence in assessment of risk

High, with some uncertainty regarding current geographic distribution of infected ticks.

Actions and recommendations

Raise local awareness on tick avoidance measures for the public and clinicians in known affected areas.

Apply serology to understand the extent of unascertained disease in humans, including among possibly exposed occupational and recreational groups, in historic undiagnosed encephalitis if possible, and through the use of research serum banks to understand background seropositivity where available.

Update the national encephalitis guidance to raise awareness among NHS staff and increase testing for TBE in clinically compatible UK-acquired encephalitis or neurological symptoms following a febrile illness. Testing should be considered where epidemiological risk exists (forest exposure or tick bite(s)), or there is no clear alternative diagnosis but compatible illness. Testing is carried out using a serological assay at the Rare and Imported Pathogens Laboratory (RIPL) in the first instance.

Consideration of whether vaccination of high-risk groups such as forestry workers is warranted at this stage, while further studies looking for evidence of human exposure or infection are undertaken.

Consideration of regionally appropriate follow-up to investigate evidence of historic and ongoing human TBEV exposure against a background of geographically varying presence of Louping ill. This work should include evidence from veterinary surveillance and research activities which have focussed principally on sheep and grouse.

In addition:

• further seroprevalence studies of deer and subsequent tick testing to identify additional potential foci of TBEV
• research into the titre of infectious virus in fresh unpasteurised milk from an infected animal
• research into the effects of cheese production processes on the quantity of infectious virus present

Step 1. Assessment of the probability of infection in the UK human population

This section of the assessment examines the likelihood of an infectious threat causing infection in the UK human population. Where a new agent is identified, there may be insufficient information to carry out a risk assessment and this should be clearly documented.

Read in conjunction with the probability algorithm found at Annexe A. There is a text alternative of Annexe A.

Is this a recognised human disease?

Outcome

Yes.

Quality of evidence

Good.

In humans, TBE is a viral infection involving the central nervous system (CNS). It is caused by TBEV, a ribonucleic acid (RNA) virus belonging to the genus Flavivirus that was initially isolated in 1937 in Russia.

Five main subtypes have been identified with differing geographic distributions:

• European or Western (TBEV-Eur)
• Siberian (TBEV-Sib)
• Far Eastern (TBEV-FE) (formerly known as Russian Spring Summer encephalitis)
• Baikalian (TBEV-Bkl)
• Himalayan (Him-TBEV) (1 to 3)

TBEV-Eur is endemic in rural and forested areas of central, eastern and northern Europe.

TBEV-FE is endemic in far-eastern Russia and in forested regions of China and Japan.

TBEV-Sib is endemic in the Urals region, Siberia and far-eastern Russia, and also in some areas in north-eastern Europe.

The recently described TBEV-Bkl found in East Siberia and Him-TBEV has been found in the Qinghai-Tibet Plateau in China (4).

In 2020 (latest full year data available), 3,817 cases (3,734 confirmed) were reported in Europe, including 16 associated fatalities (5). The highest incidence rates were reported in:

• Lithuania
• Slovenia
• the Czech Republic

Comparison of trends over a 5-year period shows that notification rates fluctuated annually and only Slovenia and Germany have seen a steady increase. In endemic regions of Europe, incidence varies considerably (6, 7).

Annual peaks in incidence are correlated with seasonal periods of increased tick activity with the majority (95%) of cases reported between May and November; the number of cases reported in a month peaked in July (1,016 cases) (5, 7).

Although cases continue to be diagnosed during the warmer months, there is no evidence of a major shift in seasonal pattern. Overall, case numbers are increasing across the region (7), associated with a variety of factors including climate change, an extended active tick season and habitat range of tick vectors, reforestation and an increase in outdoor leisure pursuits (6, 8).

Improvements in case detection and reporting may also be partly responsible for the increase (6). New foci of infections are also emerging with, for example, the Netherlands reporting TBEV for the first time in ticks and deer, and a small number of autochthonous human cases since 2016, approximately 1 to 2 cases each year (9 to 12).

Ticks are the primary route of transmission. Certain Ixodes species of tick are both reservoir and vector for TBEV, spreading the virus between wild vertebrate hosts and occasionally transmitting TBEV to humans and domesticated animals. The principal vectors are:

• Ixodes ricinus (sheep tick) for TBEV-Eur
• Ixodes persulcatus (Taiga tick) for TBEV- FE and TBEV- Sib

All tick stages can be infected, acquiring the virus from viraemic hosts during co-feeding, trans-stadially, trans-ovarially or trans-sexually (21). Ixodes ricinus (I. ricinus) is widespread in the UK, whereas Ixodes persulcatus (I. persulcatus) is considered absent.

Some human cases have been associated with consumption of unpasteurised milk or milk products from infected animals (13 to 15). TBEV is rarely transmitted from human to human via transplant (16), blood transfusion or breastfeeding (17). Animal studies have shown the potential for vertical transmission from an infected mother to the foetus (18). Infection has also been acquired accidentally in laboratories (19).

The incubation period of tick transmitted TBE is 7 days on average (a maximum of 28 days) but is shorter following foodborne transmission (approximately 4 days). The majority (around two-thirds of cases) of human TBEV infections are asymptomatic.

In clinical cases, TBE often presents as a biphasic disease. The initial viraemic phase lasts approximately 5 days (range 2 to 10 days), and is associated with non-specific symptoms such as:

• fever
• fatigue
• headache
• myalgia
• nausea

Following an asymptomatic interval of around 7 days, there is a second clinical phase involving the CNS with presentations such as meningitis, meningoencephalitis, myelitis, paralysis and radiculitis (4).

Clinical presentation and outcomes differ across the distinct subtypes of TBEV (4, 20). TBEV-Eur is associated with milder disease, with 20 to 30% of symptomatic individuals experiencing the second CNS phase, and a case fatality rate (CFR) of less than 2%. However, severe neurological sequelae are observed in up to 10% of symptomatic patients.

In children infected with TBEV-Eur, the second phase of illness is usually limited to meningitis, whereas adults aged over 40 years are at increased risk of developing encephalitis, with higher rates of mortality and long-lasting sequelae in those aged over 60 years.

TBEV-FE subtype is associated with more severe but monophasic illness, has a CFR up to 35%, and higher rates of severe neurological sequelae. TBEV-Sib subtype is associated with a less severe disease (CFR less than 3%), and a tendency for chronic or extremely prolonged infections (4). Haemorrhagic forms of the disease have been reported in the Asian part of Russia but are thought to be rare (20).

New foci of TBEV infections in ticks can appear intermittent, as suggested by a Danish study which reported the apparent absence of the virus from a previously known area (22). TBEV prevalence in ticks in endemic areas can often be less than 1%, so lack of detection in tick populations may be due to low prevalence (23). Other studies have detected the virus in ticks along with seropositive animals, without evidence of infection in humans (24). Virus prevalence in ticks does not correlate with increased risk for human infection (25).

Sustained TBEV transmission cycles are thought to require factors such as co-feeding of larvae and nymphs on small mammal hosts, due to their short viraemic period. This is considered the main reason why TBEV does not occur throughout the geographical range of its tick vectors.

Foci are thought to be limited geographically, and the areas may be as small as 0.5km² (23). Larger hosts are also important for feeding and maintaining tick populations and, although not thought to be competent for the virus, can move infected ticks to new locations (26).

Generally, in the UK, peak larval activity occurs after peak nymphal activity; however, there is some field evidence that larval and nymphal I. ricinus show coincident co-infestation on small rodents. In a woodland study in southern England, co-infestation rates of ticks on small mammals were lower than in TBEV endemic parts of Europe, but it does demonstrate that co-infestation is possible in the UK and therefore tick-to-tick transmission could occur (27).

Recent evidence of TBEV transmission in the Netherlands highlights that climatic and other environmental factors may have an impact on changing viral distribution to parts of western Europe.

Is the disease endemic in the UK?

Outcome

Yes – in ticks in high risk areas.

No – not ubiquitous.

Quality of evidence

Poor.

Travel-related TBE cases are occasionally diagnosed in the UK – 7 confirmed cases were reported between 2014 and 2018 (28).

Although no confirmed autochthonous human cases (based on EU case definition) have been reported in the UK to date, since 2019, 2 probable (based on serology) and 2 PCR-positive cases of TBEV human infection have been reported in England, one of which was likely to have been acquired in Scotland.

In July 2019, a probable case of TBEV diagnosed through serology alone was reported in a German infant who had travelled to the New Forest (Hampshire) and acquired a tick bite (29).

In July 2020, a second probable TBE case was reported in a patient who lives in Hampshire and recently acquired a tick bite locally (45). These cases were diagnosed based on serological testing, and compatible clinical and exposure histories.

A third case, positive by PCR, was reported in England in September 2022, with infection likely to have been acquired in Scotland in June 2022. A fourth case, also positive by PCR, was reported in England in October 2022, with a likely exposure while visiting the North Yorkshire Moors.

Louping ill virus (LIV), a related virus that displays a high degree of genetic homology to TBEV, is present in areas of the UK and can complicate surveillance for TBEV in animals and humans due to serological cross-reactivity between the 2 viruses. However, LIV can be distinguished from other TBE complex viruses by PCR and/or whole genome sequencing.

The TBEV vector I. ricinus is a common tick found throughout the British Isles in woodlands, grazed grasslands, moor-land, heath-land and some urban parks, with evidence of recent expansion in its range in the UK based upon UKHSA tick surveillance data (30, 46, 47). Across Europe, although both TBEV (31) and I. ricinus have a wide distribution, TBE incidence is variable and does not occur in all areas where the tick is found.

In 2019, research conducted in England and Scotland detected evidence of TBEV for the first time through investigating deer seroprevalence, and by testing ticks (32). During 2018, approximately 1,300 deer serum samples from England and Scotland were tested by TBEV IgG ELISA and LIV haemagglutination inhibition assay (HAI). Blood-fed ticks from deer in areas with seropositive deer were tested by reverse transcription polymerase chain reaction (RT-PCR) using a LIV/TBEV RNA assay, and a secondary LIV specific assay. Five ticks from the Thetford Forest area were TBEV PCR positive and a full-length genome of TBEV was obtained by sequencing from one of these.

Phylogenetic analysis confirmed the sequence to belong to the TBEV-Eur subtype; it is most closely related to the Norwegian Mandal strain of TBEV isolated from ticks in 2009 sharing a 99% sequence identity. Subsequent surveys of questing ticks in Thetford Forest were conducted, and collected ticks pooled. A small number of tick pools were positive. A map of locations for seropositive deer and positive ticks removed from deer is available (32).

Subsequently, the presence of TBEV was detected in a questing tick pool in southern England in September 2019 at a location on the Hampshire and Dorset border (33). A small number of the questing tick pools collected 2020 in the New Forest were also positive. This virus is most closely related to TBEV-NL (LC171402.1), a strain of TBEV detected in ticks in the Netherlands in 2017.

The findings of TBEV in questing ticks in 2 parts of the country are consistent with the areas of higher deer seropositivity. This suggests that there is evidence of enzootic transmission of TBEV in certain foci in England. Ongoing serosurveillance of deer in 2020 for LIV/TBEV has again found the highest seropositivity in Thetford Forest (Norfolk and Suffolk) and New Forest (Hampshire).

Are there routes of introduction into the UK?

Outcome

Yes.

Quality of evidence

Good.

In 2019, TBEV was found for the first time in ticks (questing and on deer) in the East of England, and then in ticks (questing) in Hampshire and Dorset (32, 33). The detected viruses are of different lineages, indicating that at least 2 independent incursions have occurred. Subsequently, TBEV positive ticks have been found in North Yorkshire.

There is potential for migratory birds to introduce TBEV infected ticks into the UK from endemic areas, as I. ricinus ticks infected with TBEV have been detected on migratory birds in Europe (34). The virus has not been detected from tick samples taken from migratory birds in the UK (35).

There is also a possibility of TBEV introduction through animal movement, including companion animals, inadvertently transporting infected ticks. Changes in the PETS travel scheme (52) in 2012 which removed compulsory tick treatment of companion animals entering the UK from Europe increased this risk.

Are effective control measures in place to mitigate against these routes of introduction?

Outcome

No.

Quality of evidence

Good.

Prevention of the introduction of infected ticks through animal movement would be necessary to mitigate the risk of TBEV introduction by that route. However, it is impossible to prevent introduction of ticks via migratory birds.

The revised PETS travel scheme does not require the compulsory treatment of pets for ticks prior to return or entry to the UK (52). In recent years, public and animal health has focussed on education and awareness raising of this increased risk of importation of ticks with both veterinary professionals (via publications in veterinary journals and publications) and the general public (UKHSA tick poster).

However, given the continued submissions of ticks from imported pets to the UKHSA Tick Surveillance Scheme (36), it can be presumed that imported ticks on pets, including those that have potentially travelled from or through TBEV endemic countries, will continue to present a risk of introduction of TBEV to the UK.

Do environmental conditions in the UK support the natural reservoirs or vectors of disease?

Outcome

Yes.

Quality of evidence

Good.

I. ricinus, both a reservoir and the vector of TBEV, is present and abundant throughout the UK (30).

Transmission of TBEV is highly reliant on co-feeding of nymphs and larvae, and a recent study has shown some evidence of co-infestation (27).

Climate change models also suggested a northern spread of TBEV in Europe (37).

Will there be human exposure?

Outcome

Yes – high risk groups.

No – general population.

Quality of evidence

Satisfactory.

To date, 2 probable cases of TBEV human infections have been reported in Hampshire, England. A further case, positive by PCR, is likely to have acquired infection in Scotland.

Since infected ticks have been found in defined areas in Thetford Forest and Hampshire and Dorset, there is the potential for ongoing human exposure. Additionally, a PCR-confirmed human TBEV case was reported who had visited North Yorkshire, in a similar area to where 3 TBEV positive tick pools were collected.

With these findings, at-risk groups for potential human exposure to TBEV infected ticks may now include those visiting, living or working in areas where infected ticks are present in the UK, and certain occupational groups may be at increased risk. Risk areas elsewhere in Europe are usually geographically limited due to co-feeding transmission that occurs on a small scale, sometimes as small as 0.5km² (23). Exposure to infected ticks in the UK will likely be limited to similarly small foci.

There are farmed livestock of varied species on many premises within and around Thetford Forest and New Forest areas (Defra unpublished data). The number of dairy holdings (bovine and caprine) close to these areas registered for raw milk production is low, possibly 4 (FSA unpublished data). There is a risk of TBE through the food chain via unpasteurised drinking milk (48), or cheese made from unpasteurised milk (49, 50).

The FSA has assessed the risk of infection with TBEV to consumers in these areas (51), as follows:

• from drinking unpasteurised milk from ruminants affected by TBE to be Very Low to Low with medium uncertainty
• from cheese made from unpasteurised milk to be Negligible to Very Low with medium uncertainty
• from pasteurised dairy products to be Negligible with low uncertainty
• from consuming meat from animals affected by TBE to be Negligible to Very Low with high uncertainty

The Advisory Committee on the Microbiological Safety of Food noted when assessing the above FSA risk assessment that it would be sensible to mark this subject for revisiting in the future when more data is available, potentially covering a broader geographical area and a longer time period.

Comprehensive information on tick avoidance is on the GOV.UK website. This advice was produced primarily for Lyme disease, but is also applicable given TBEV is also transmitted by I. ricinus. Individuals can avoid exposure by using barrier methods when handling infested animals or entering tick infested environments. Protective measures include avoiding tick habitats, wearing long sleeves and trousers, using tick repellent or impregnated clothing, and checking frequently for ticks (38, 39).

Are humans highly susceptible?

Outcome

No.

Quality of evidence

Good.

Humans are susceptible to TBEV infection. Most (two-thirds) infections are asymptomatic. TBEV-Eur is associated with milder disease than the other subtypes, with 20% to 30% of patients experiencing the second CNS phase and severe neurological sequelae is observed in up to 10% of patients. All age groups are susceptible but individuals of older age or with existing chronic conditions may be at higher risk of mortality and longer-term sequelae (38), and morbidity in children can be significant (20).

In TBEV endemic countries, a position paper by the World Health Organization (WHO), suggests that TBEV vaccine should be offered to all age groups in highly endemic areas; those with incidence rates above 5 per 100,000 (40). Due to the potential for high morbidity in children, many countries recommend vaccination of children in TBEV endemic areas (6).

In the UK, a licensed TBE vaccine is available and is currently recommended only for those ‘at high risk of exposure to the virus’, through travel to endemic areas or employment (41 to 43). The Joint Committee on Vaccination and Immunisation (JCVI) has been asked to consider whether vaccination of high-risk groups such as forestry workers is warranted at this stage, while further studies looking for evidence of human exposure or infection are undertaken.

Outcome of probability assessment

Assessment of the probability of human infection with TBEV in the UK population.

Probability

Very low for the general population.

Low for high risk groups (defined risk areas only).

Step 2. Assessment of the impact on human health

The scale of harm caused by the infectious threat in terms of morbidity and mortality depends on spread, severity, availability of interventions and context.

Read in conjunction with the impact algorithm following the boxes shaded green found in Annexe B. There is a text alternative of Annexe B.

Is there human-to-human spread of this pathogen?

Outcome

No.

Quality of evidence

Good.

TBEV is not directly transmitted from human to human, except in very rare cases via organ transplantation, blood transfusion, breastfeeding or the transplacental route (16, 17).

Human exposure to TBEV-Eur is primarily through the bite of an infected tick (I. ricinus in Europe) although foodborne transmission is occasionally reported (13 to 15). Accidental laboratory transmission has been reported (19).

Is there zoonotic or vector-borne spread of this pathogen?

Outcome

Yes.

Quality of evidence

Good.

I. ricinus is the primary vector for the TBEV-Eur transmission to humans, although foodborne transmission (mainly through contaminated unpasteurised milk) is occasionally reported (13 to 15).

For zoonoses or vector-borne disease, is the animal host or vector present in the UK?

Outcome

Yes.

Quality of evidence

Good.

I. ricinus, the tick vector for TBEV-Eur, is present and abundant in the UK (30). Small mammals (for example, rodents) that are able to support co-feeding transmission and large mammals (for example, sheep, goat, roe deer), which serve as important hosts for maintaining tick populations are also present in UK.

Is the UK human population susceptible?

Outcome

Yes.

Quality of evidence

Good.

Approximately two-thirds of human TBEV infections are subclinical, but the clinical spectrum ranges from mild disease (non-specific febrile illness) to CNS involvement (for example, meningitis, severe meningoencephalitis with or without paralysis). Symptomatic infection can occur in all age groups, and is often more severe in adults, especially the elderly. The TBEV-Eur subtype is associated with milder disease compared to the other 2 virus subtypes.

TBE typically follows a biphasic course; a viraemic phase with flu-like symptoms, followed by a period of quiescence, then the second phase with CNS involvement.

Approximately a third of patients experience the second phase, and up to 20% of those with severe disease experience neurological sequelae. According to a 10-year follow-up survey, 80% of patients with primary myelitic disease will remain with sequelae (44). Overall, the mortality rate is 0.5% to 2% (4).

Would a significant number of people be affected?

Outcome

No.

Quality of evidence

Good.

The vast majority of TBEV infections are acquired by tick bite, thus only those who are exposed to and bitten by infected ticks will be affected. To date, TBEV has only been found in defined areas of eastern and southern England, with 2 further probable human cases that are likely to have acquired their infections in the Loch Earn area of Scotland and North Yorkshire Moors. Exposure would be limited to those living in, working in or visiting those areas.

While the vector I. ricinus is a common tick found throughout the British Isles, even in endemic countries, the rate of infection is relatively low, for example:

• Germany: 0.8/100,000
• Austria: 2.8/100,000
• Lithuania: 24.3/100,000
  (28)

Are effective interventions (preventative or therapeutic) available?

Outcome

Yes.

Quality of evidence

Good.

Individuals can avoid tick bites by keeping skin covered as much as possible when visiting a tick-infested area and using effective insect repellents. Individuals should check their body for ticks regularly when visiting tick-infected areas, with any ticks identified being removed as soon as possible with a pair of fine tipped tweezers or tick remover. Consumption of unpasteurised dairy products should also be avoided in endemic areas. An effective and well-tolerated vaccination is available for protection against TBE and has been introduced into childhood immunisation schedules in endemic areas in Europe.

There is no specific treatment for TBE. Supportive treatment can significantly reduce morbidity and mortality.

In the UK, a licensed TBE vaccine is available and is currently recommended only for those ‘at high risk of exposure to the virus’, through travel to endemic areas or employment (41 to 43). The JCVI has been asked to consider whether vaccination of high-risk groups such as forestry workers is warranted at this stage, while further studies looking for evidence of human exposure and infection are undertaken.

Outcome of impact assessment

The impact of TBE virus on human health in the UK is Low.

Annexe A. Assessment of the probability of infection in the UK population algorithm

Where the evidence may be insufficient to give a definitive answer to a question, the alternative is also considered. The most likely outcome is shown in solid colour and the alternative outcome is shown in hatched colour.

Summarised text version of Annexe A

Outcomes are specified with (Outcome) beside the appropriate answer.

Question 1. Is this a recognised human disease?

Yes

Go to question 3. (Outcome)

No

Go to question 2.

Question 2. Is this a zoonosis or is there a zoonotic potential?

Yes

Go to question 4.

No

The probability of infection in the UK population is considered very low.

Question 3. Is this disease endemic in humans within the UK?

Yes [note 1]

Go to question 5.

No

Go to question 4. (Outcome)

[note 1] This pathway considers reverse-zoonosis of a pathogen already in circulation in the human population.

Question 4. Is this disease endemic in animals in the UK?

Yes

In tick populations in identified risk areas. Go to question 8. (Outcome)

No

It is not ubiquitous across the UK. Go to question 5. (Outcome)

Question 5. Are there routes of introduction into animals in the UK?

Yes

Go to question 6. (Outcome)

No

The probability of infection in the UK population is considered very low.

Question 6. Are effective measures in place to mitigate against these?

Yes

The probability of infection in the UK population is considered very low.

No

Go to question 7. (Outcome)

Question 7. Do environmental conditions in the UK support the natural vectors of disease?

Yes, or unknown

Go to question 8. (Outcome)

No

The probability of infection in the UK population is considered very low.

Question 8. Will there be human exposure?

Yes (high-risk groups)

Go to question 9. (Outcome)

No (general population)

The probability of infection in the general UK population is considered very low. (Outcome)

Question 9. Are humans highly susceptible? [note 2]

Yes

Go to question 10.

No

The probability of infection in high-risk groups within the UK population is considered low. (Outcome)

[note 2] Includes susceptibility to animal-derived variants

Question 10. Is the disease highly infectious in humans?

Yes

The probability of infection in the UK population is considered high.

No

The probability of infection in the UK population is considered moderate.

Annexe B. Assessment of the impact on human health algorithm

Where the evidence may be insufficient to give a definitive answer to a question, the alternative is also considered. The most likely outcome is shown in solid colour and the alternative outcome in hatching.

Summarised text version of Annexe B

Outcomes are specified with (Outcome) beside the appropriate answer.

Question 1. Is there human-to-human spread?

Yes

Go to question 4.

No

Go to question 2. (Outcome)

Question 2. Is there zoonotic or vector-borne spread?

Yes

Go to question 3. (Outcome)

No

The impact of infection in the UK population is considered very low.

Question 3. Is this disease endemic in humans within the UK?

Yes

Go to question 4. (Outcome)

No

The impact of infection in the UK population is considered very low.

Question 4. Is the human population susceptible?

Yes

Go to question 5. (Outcome)

No

The impact of infection in the UK population is considered very low.

Question 5. Does it cause severe disease in humans?

Yes

Go to question 8. (Outcome)

No

Go to question 6.

Question 6. Is it highly infectious to humans?

Yes

Go to question 9.

No

Go to question 7.

Question 7. Are effective interventions available?

Yes

The impact of infection in the UK population is considered very low.

No

The impact of infection in the UK population is considered low.

Question 8. Would a significant [note 1] number of people be affected?

Yes

Go to question 10.

No

Go to question 9. (Outcome)

[note 1] This question has been added to differentiate between those infections causing severe disease in a handful of people and those causing severe disease in larger numbers of people. ‘Significant’ is not quantified in the algorithm but has been left open for discussion and definition within the context of the risk being assessed.

Question 9. Are effective interventions available?

Yes

The impact of infection in the UK population is considered low. (Outcome)

No

The impact of infection in the UK population is considered moderate.

Question 10. Is it highly infectious to humans?

Yes

Go to question 12.

No

Go to question 11.

Question 11. Are effective interventions available?

Yes

The impact of infection in the UK population is considered moderate.

No

The impact of infection in the UK population is considered high.

Question 12. Are effective interventions available?

Yes

The impact of infection in the UK population is considered high.

No

The impact of infection in the UK population is considered very high.

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