Independent report

Appendix 1: estimation of number needed to vaccinate to prevent a COVID-19 hospitalisation for primary vaccination, booster vaccination (3rd dose), autumn 2022 and spring 2023 booster for those newly in a risk group

Updated 27 January 2023

Based on a UK Health Security Agency (UKHSA) presentation to the Joint Committee on Vaccination and Immunisation (JCVI) on 25 October 2022.

In order to consider the benefits of continued offers of primary vaccination, booster vaccination, the autumn booster, and potential boosting in the spring of 2023 (for example, for those newly entering a risk group) calculation of numbers needed to vaccinate (NNV) were completed. To do this calculation, the following information was used:

• rates of COVID-19 and severe COVID-19 hospitalisation per million population in July 2022 by vaccination status. ‘Severe’ was taken to mean there were codes to indicate use of oxygen or ventilation and admission to intensive care (ICU)
• vaccine effectiveness for 2 doses and 2+1 doses by time since the last dose against COVID-19 hospitalisation and against severe COVID-19 hospitalisation

To estimate the rates of hospitalisation, patients with a respiratory discharge diagnosis as the main diagnosis who also tested positive for SARS-CoV-2 and had at least a 2-day hospital stay, were identified from the secondary user service (SUS) for admissions in July 2022. This was the most recent month where data should be near complete. Rates in this month will reflect rates according to the recent immune status of the population. Using only those with a respiratory code as the main diagnosis and 2 days stay aligns the rates with the outcomes used for vaccine effectiveness and is substantially lower than all hospitalisations with a recent positive test which can include many individuals hospitalised with, rather than because of, COVID-19.

The data were linked to the national immunisation register (NIMS) to allow rates to be estimated by vaccination status (unvaccinated, 1 or 2 doses, 3 or more doses). The NIMS data also had data on whether individuals were in a clinical risk group (for adults) so could be used to stratify rates by risk group. The rates used are given in Table 1. For the NNV calculation it was assumed these rates would apply for the year following vaccination.

Table 1a: rates per million of COVID-19 hospitalisation by age and risk group (for ages 20 to 59). Rates are for July 2022

Age (years)	0 doses (all)	1 or 2 doses (all)	3+ doses (all)	0 doses (clinical risk group)	1 or 2 doses (clinical risk group)	3+ doses (clinical risk group)	0 doses (not clinical risk group)	1 or 2 doses (not clinical risk group)	3+ doses (not clinical risk group)
5 to 11	4.2	3.4	-	-	-	-	-	-	-
12 to 15	4.6	2.4	0.0	-	-	-	-	-	-
16 to 19	12.9	2.5	3.9	-	-	-	-	-	-
20 to 29	10.9	10.9	7.0	60.1	56.3	38.3	7.3	5.7	1.7
30 to 39	14.6	12.6	8.0	93.4	62.7	36.7	6.7	3.6	1.4
40 to 49	14.5	20.1	13.8	66.7	76.8	47.4	6.7	4.3	3.1
50 to 59	47.6	63.7	35.8	174.5	164.5	93.4	13.3	12.2	6.6
60 to 69	119.0	183.9	80.5	-	-	-	-	-	-
70 +	414.5	382.3	342.7	-	-	-	-	-	-

Table 1b: rates per million of severe COVID-19 hospitalisation by age and risk group (for ages 20 to 59). Rates are for July 2022

Age (years)	0 doses (all)	1 or 2 doses (all)	3+ doses (all)	0 doses (clinical risk group)	1 or 2 doses (clinical risk group)	3+ doses (clinical risk group)	0 doses (not clinical risk group)	1 or 2 doses (not clinical risk group)	3+ doses (not clinical risk group)
5 to 11	1.1	1.7	-	-	-	-	-	-	-
12 to 15	0.8	0.6	0.0	-	-	-	-	-	-
16 to 19	1.2	0.8	1.3	-	-	-	-	-	-
20 to 29	0.8	0.4	0.9	10.9	3.8	4.0	0.0	0.0	0.3
30 to 39	1.4	0.9	1.1	11.7	5.7	5.9	0.4	0.0	0.0
40 to 49	2.3	4.0	1.4	13.3	15.4	4.8	0.7	0.9	0.3
50 to 59	6.7	10.1	5.0	22.4	26.9	12.9	2.4	1.5	0.9
60 to 69	21.8	17.8	8.8	-	-	-	-	-	-
70 +	50.9	15.6	32.0	-	-	-	-	-	-

Note that the rates given in Table 1a and 1b are crude rates by age and vaccination status and should not be compared with one another to infer vaccine effectiveness. Rates will be affected by previous infections and other differences between groups. For example, those unvaccinated are likely to have had higher prior infection rates than those vaccinated which can reduce recent incidence in this group.

The effectiveness of primary vaccination and doses 3 and 4 by time since vaccination used in the calculations are given in Table 2. These are based on estimated effectiveness of vaccination against the Omicron variant - see COVID-19 vaccine surveillance report: 4 November 2021 (week 44). Note that when considering the effect of doses 3 and 4 it is assumed that there is residual long-term protection as shown in the table at the longer intervals after these doses. Therefore, for example, the benefit of a booster (dose 3) is to increase effectiveness from the waned second dose level to the third dose level (for example, from 50% to 90%).

Table 2: vaccine effectiveness estimates (%) used in the calculations

Month	Primary (hospitalisation)	Booster (dose 3) (hospitalisation)	Booster (dose 4) (hospitalisation)	Primary (severe hospitalisation)	Booster (dose 3) (severe hospitalisation)	Booster (dose 4) (severe hospitalisation)
1	0	80	80	0	85	85
2	60 (1 dose)	90	90	70 (1 dose)	95	95
3	60 (1 dose)	90	90	70 (1 dose)	95	95
4	80	80	80	90	90	90
5	80	80	80	90	90	90
6	80	70	70	90	80	80
7	60	70	70	70	80	80
8	60	60	60	70	70	70
9	60	60	-	70	70	-
10	50	60	-	60	70	-
11	50	60	-	60	70	-
12	50	60	-	60	70	-

The time horizon for benefits was taken to be a year following vaccination. To estimate cases prevented per million doses (or courses of 2 doses for primary vaccination) the effectiveness in each month (Table 2) was applied to the relevant rates from Table 1. For example, for assessing the booster (dose 3) against hospitalisation 3 months after it is given in 60 to 69 year olds, we use the rate of 183.9 per million from Table 1 as the non-boosted rate and use the effectiveness of a booster after 3 months (90%) compared to waned effectiveness of 2 doses of 50% to give a relative effectiveness of 80% (note 0.8 = 1 – (1-0.9)/(1-0.5)). An 80% reduction on 183.9 means 147.1 prevented from 1 million vaccinated. The prevented cases are summed over the 12 months to give a total prevented per million doses and this is then inverted to give NNV to prevent a case.

The results of NNV by age and clinical risk groups are shown in Tables 3 and 4. Strong age effects are seen as well as large differences within adults ages by clinical risk group. The oldest age groups and the older individuals in clinical risk groups have the lowest NNV. There are also higher NNVs when moving from primary to each booster dose.

Table 3a: NNV for prevention of hospitalisation

Early discussions on a possible spring 2023 programme focussed on risk groups and therefore the NNV calculations were only done for risk groups – see table 3b.

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster
5 to 11	34,200	-	-
12 to 15	31,400	-	-
16 to 19	11,200	76,000	73,500
20 to 29	13,300	17,600	40,900
30 to 39	9,900	15,300	35,900
40 to 49	10,000	9,600	20,600
50 to 59	3,000	3,000	8,000
60 to 69	1,200	1,000	3,600
70+	300	500	800

Table 3b: NNV for prevention of hospitalisation for patients in a risk group

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster	Spring 2023 booster
20 to 29	2,400	3,400	7,500	7,500
30 to 39	1,600	3,100	7,800	7,800
40 to 49	2,200	2,500	6,000	6,000
50 to 59	800	1,200	3,100	3,100

Table 3c: NNV for prevention of hospitalisation for patients in no risk group

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster
20 to 29	19,900	33,900	168,200
30 to 39	21,700	53,800	210,400
40 to 49	21,700	44,900	92,500
50 to 59	10,900	15,800	43,600

Table 4a: NNV for prevention of severe hospitalisation

Early discussions on a possible spring 2023 programme focussed on risk groups and therefore the NNV calculations were only done for risk groups – see table 4b.

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster
5 to 11	112,200	-	-
12 to 15	162,600	-	-
16 to 19	106,500	193,500	185,100
20 to 29	166,200	418,100	275,200
30 to 39	87,600	188,500	217,300
40 to 49	53,700	40,600	175,900
50 to 59	18,700	16,200	48,300
60 to 69	5,700	9,200	27,300
70+	2,500	10,400	7,500

Table 4b: NNV for prevention of severe hospitalisation for patients in a risk group

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster	Spring 2023 booster
20 to 29	11,400	43,500	59,500	59,500
30 to 39	10,700	28,600	40,500	40,500
40 to 49	9,400	10,600	49,800	49,800
50 to 59	5,600	6,100	18,600	18,600

Table 4c: NNV for prevention of severe hospitalisation for patients in no risk group

Age (years)	Primary	Booster (2+1)	Autumn 2022 booster
20 to 29	No cases	No cases	706,500
30 to 39	318,400	No cases	No cases
40 to 49	186,800	190,400	932,500
50 to 59	51,600	10,700	256,400