Independent report

Appendix A: estimating the number needed to vaccinate to prevent a COVID-19 hospitalisation in autumn 2024 in England

Published 2 August 2024

Introduction

In this report the UK Health Security Agency (UKHSA) estimates the number needed to vaccinate (NNV) to prevent a hospitalisation, severe hospitalisation and a death due to COVID-19. This is to inform decisions around COVID-19 vaccination in autumn 2024. The data relevant to these calculations is incidence of these outcomes by vaccination status as well as incremental vaccine effectiveness of additional doses and how this wanes. Stratification by risk group is shown using data in the National Immunisation Management System (NIMS) and this considers risk as those with and without immunosuppression. The analysis focuses on ages 15 and over and stratifies by age in 5-year bands to over 90 years.

The method is the same as that used for autumn 2023 with updates to estimates of vaccine effectiveness and a correction factor for the lower incidence seen since winter 2022 to 2023.

Methods

Outcomes

The outcomes were:

• hospitalisations: secondary user services (SUS) hospital admissions with a respiratory code in the primary discharge field and with a positive PCR or lateral flow test in the period 14 days before to 2 days after admission. In addition, any individuals without evidence of a test but with an International Classification of Diseases (ICD) code for COVID (U071-COVID virus identified and U072-COVID epi diagnosis) in the primary diagnostic field
• severe hospitalisations: at least a 2-day stay and with codes to indicate use of oxygen, ventilation or ICU admission
• death: Office of National Statistics (ONS) coded deaths with COVID-19 listed as one of the causes

Covariates

Age was defined in 5-year age bands from 15 years of age. Clinical risk group was defined as having a flag for any of the risk group fields in NIMS as follows:

• no risk: no risk group flags
• risk with no immunosuppression: at least one risk group flag but not a flag for immunosuppression
• immunosuppressed: a flag for immunosuppression or severely immunosuppressed

The full list of flags followed the Cohorting as a Service (CaaS) (created autumn 2022 for the booster) cohorts:

• asplenia
• chronic heart or vascular disease
• chronic kidney, liver or digestive disease
• chronic neurological disease
• chronic respiratory disease
• diabetes and endocrine disease
• morbid obesity
• severe mental illness
• serious genetic abnormalities
• immunosuppression
• severely immunosuppressed

The latter 2 are used for immunosuppression.

Vaccine effectiveness of further doses

To estimate the incremental effectiveness (iVE) of a further vaccine dose, the recent VE estimates of the autumn 2023 doses were used along with data from the spring 2023 dose for which longer follow up is available (in those not boosted again) to assess waning. This allowed estimates of iVE by month since a further dose with a 6-month window.

The VE estimates based on the test-negative case-control (TNCC) run on 14 February 2024 for the hospital end point with at least a 2-day stay were as follows:

• 9 to 13 days post vaccination: 41% (95% CI 28 to 52)
• 2 to 4 weeks post bivalent vaccine: 51% (44 to 56)
• 5 to 9 weeks: 46% (39 to 501)
• 10 to 14 weeks: 37% (29 to 43)
• greater than 15 weeks: 14% (−12 to 33)

See recent published estimates

Based on this and data from the spring 2023 boosters the iVE by month since booster was assumed to be as follows for all end points:

• month 1: 45%
• month 2: 45%
• month 3: 35%
• month 4: 20%
• month 5: 15%
• month 6: 10%

Method to estimate NNV

To estimate NNV the monthly incidence was based on the incidence used for the autumn 2023 calculation (incidence in the winter of 2022 to 2023) but with this incidence halved to reflect the lower rates of COVID-19 seen since this time through 2023 which likely reflect increasing population immunity. As in the 2023 calculation, incidence is stratified by age and risk group.

The NNV was then calculated by applying the iVE to this incidence for a 6-month period post a booster dose. For example, an incidence of 10 per 100,000 per month would be reduced with vaccination by:

• 10 times 0.45 equals 4.5 in month 1
• 10 times 0.45 equals 4.5 in month 2
• 10 times 0.35 equals 3.5 in month 3
• 10 times 0.2 equals 2 in month 4
• 10 times 0.15 equals 1.5 in month 5
• 10 times 0.1 equals 1 in month 6

This accumulates to observing 17 per 100,000 population fewer cases which can be inverted to an NNV of 100,000 divided by 17 equals 5,882.

Results

NNV estimates

NNV estimates are shown in tables 1a to 1c. As expected NNV decreases with increasing risk and age. It is notable that NNV is lower for death than for severe hospitalisation. This likely relates to deaths occurring outside hospital and possibly older ages in hospital dying before oxygen, ventilation or ICU use. It may also be that some ONS coded deaths were not due to COVID-19.

Notes on tables 1a to 1c:

• ‘IS’ means immunosuppressed
• ‘all’ means combining no risk and risk groups

Table 1a: NNV estimates for a hospitalisation by age and risk status

Age (years)	No risk	Risk (not including IS)	IS	All
15 to 19	113,700	9,800	1,100	63,500
20 to 24	102,900	10,200	1,300	69,900
25 to 29	93,800	10,500	1,500	62,100
30 to 34	86,700	10,700	1,700	56,000
35 to 39	82,100	10,700	1,800	50,800
40 to 44	80,000	10,400	1,900	45,100
45 to 49	80,200	9,600	1,900	37,100
50 to 54	79,700	8,600	1,800	15,500
55 to 59	75,000	7,200	1,600	12,300
60 to 64	63,700	5,700	1,400	9,000
65 to 69	46,700	4,200	1,200	6,000
70 to 74	28,500	2,900	910	3,800
75 to 79	14,900	1,900	690	2,200
80 to 84	7,000	1,200	500	1,300
85 to 89	3,000	690	360	730
over 90	1,200	410	260	420

Table 1b: NNV estimates for a severe hospitalisation by age and risk status 

Age (years)	No risk	Risk (not including IS)	IS	All
15 to 19	>5,000,000	416,900	97,700	>5,000,000
20 to 24	>5,000,000	316,100	89,700	>5,000,000
25 to 29	>5,000,000	240,400	81,500	3,974,100
30 to 34	>5,000,000	183,900	72,500	2,428,400
35 to 39	3,793,800	141,800	62,500	1,517,400
40 to 44	2,405,300	110,600	51,700	952,200
45 to 49	1,730,200	87,300	40,800	579,100
50 to 54	1,347,400	69,400	31,000	180,000
55 to 59	1,067,900	55,100	23,000	124,700
60 to 64	809,700	43,400	16,900	83,300
65 to 69	552,000	33,600	12,500	54,600
70 to 74	323,300	25,500	9,400	35,200
75 to 79	165,500	18,900	7,100	22,600
80 to 84	76,600	13,900	5,500	14,700
85 to 89	33,100	10,100	4,300	10,000
over 90	13,900	7,300	3,300	7,000

Table 1c: NNV estimates for death by age and risk status

Age (years)	No risk	Risk (not including IS)	IS	All
15 to 19	4,859,000	3,837,300	53,800	3,497,000
20 to 24	4,406,600	1,812,900	61,300	3,473,500
25 to 29	3,970,400	879,200	68,200	2,913,800
30 to 34	3,531,300	449,400	72,300	2,317,000
35 to 39	3,080,200	248,500	71,100	1,719,600
40 to 44	2,617,900	152,600	63,400	1,168,300
45 to 49	2,145,200	105,400	50,400	708,200
50 to 54	1,649,700	78,400	36,100	206,900
55 to 59	1,154,100	59,400	23,900	132,900
60 to 64	712,100	43,300	14,900	79,200
65 to 69	375,600	28,700	8,900	43,300
70 to 74	166,100	16,600	5,300	21,500
75 to 79	63,200	8,500	3,000	9,900
80 to 84	21,500	4,000	1,700	4,300
85 to 89	6,800	1,700	1,000	1,900
over 90	2100	740	570	780

Comments

Assumptions in this calculation include:

• booster iVE wanes back to the pre-booster protection level by the time of future doses. In practice some incremental immunity may remain for those revaccinated at about 6 months. This will lead to slight under estimation of NNV
• the same iVE applies to all risk groups (and those not in a risk group)

• half the incidence assumed for autumn 2023 is a best approximation of future rates based on lower rates seen in 2023. A reasonable uncertainty would be doubling to halving the rates (and hence NNV)

• new variants will not change the iVE appreciably