Risk of Infection and Hospitalization Among Vaccinated and Unvaccinated Children and Adolescents in New York After the Emergence of the Omicron Variant

Abstract

This database study examines vaccination for COVID-19 among children in New York after the emergence of the Omicron variant, including analysis of case rates and hospitalizations for COVID-19 disease.

In the US, 12.3 million youth aged 18 years and younger were diagnosed with COVID-19 by April 7, 2022.¹ Studies conducted before the Omicron variant’s emergence indicated that the BNT162b2 vaccine is safe and effective in preventing COVID-19 outcomes in persons aged 5 years and older.^2,3,4,5 Compared with adolescents aged 12 to 17 years (30-μg doses), less is known about vaccination outcomes for children aged 5 to 11 years (10-μg doses), who were fully vaccinated only after the emergence of Omicron.

Methods

Linking 4 New York state databases for COVID-19 vaccinations, cases, and admissions, this analysis compared 2 outcomes among fully vaccinated (≥14 days after primary series completion) vs unvaccinated youth (partially vaccinated excluded) in the age groups 5 to 11 and 12 to 17 years: cases, defined as positive nucleic acid amplification test or antigen results; and hospitalizations (eMethods in the Supplement).

Cases and hospitalizations were enumerated for weekly cohorts of fully vaccinated and unvaccinated individuals for each age group from November 29, 2021, to January 30, 2022. Within each age group, incidence rate ratios (IRRs) were calculated, comparing unvaccinated vs vaccinated rates.

Case rates for youth achieving fully vaccinated status between December 13, 2021, and January 2, 2022, were estimated by time since vaccination across the period including January 3 through 30, 2022, and compared with unvaccinated with IRR during each period, separately for each age group (eFigure in the Supplement).

Statistical significance of IRRs was evaluated with exclusion of 1 in 95% CIs (eMethods in the Supplement). Boosted adolescents were included in weekly cohorts but not analyses of time since vaccination. Analyses were conducted with SAS version 9.4 (SAS Institute), with 95% CIs estimated with OpenEpi and Excel. The New York State Department of Health institutional review board determined this surveillance activity was exempt from review and the need for patient consent.

Results

As of January 30, 2022, 365 502 children aged 5 to 11 years were fully vaccinated (mean [SD] age, 8.3 [2.0] years; 51% boys) and 997 554 were unvaccinated (mean [SD] age, 7.8 [2.0] years; 51% boys). Among adolescents aged 12 to 17 years, 852 384 were fully vaccinated (mean [SD] age, 14.6 [1.6] years; 50% boys) and 208 145 were unvaccinated (mean [SD] age, 14.6 [1.7] years; 53% boys). During observation, 140 680 cases of COVID-19 and 414 hospitalizations were observed among children aged 5 to 11 years; among adolescents aged 12 to 17 years, there were 154 555 cases and 671 hospitalizations (Table).

Table. New COVID-19 Cases and Hospitalizations by Vaccine Status, Children Aged 5-17 Years in New York State, November 29, 2021, Through January 30, 2022^a.

Week, by patient age group	No. of events		Rates per 100 000		Incidence rate ratio (95% CI)	Full vaccine coverage, %	% of sequences Omicronb
	Vaccinated	Unvaccinated	Vaccinated	Unvaccinated
Cases
5-11 yc
Dec 13-19	204	10 062	39	122	3.1 (2.7-3.6)	4.7	19.2
Dec 20-26	1051	16 915	91	212	2.3 (2.2-2.5)	10.6	55.7
Dec 27–Jan 2	2910	28 476	184	369	2.0 (1.9-2.1)	14.4	84.7
Jan 3-9	4535	36 256	251	484	1.9 (1.9-2.0)	16.5	90.1
Jan 10-16	3459	18 647	170	256	1.5 (1.5-1.6)	18.5	95.6
Jan 17-23	2403	9289	105	131	1.3 (1.2-1.3)	20.9	97.9
Jan 24-30	1584	4889	62	70	1.1 (1.1-1.2)	23.4	99.2
12-17 y
Nov 29–Dec 5	848	3336	15	101	6.7 (6.2-7.2)	58.5	0
Dec 6-12	1302	4245	23	131	5.7 (5.3-6.0)	58.9	2.1
Dec 13-19	4140	6738	73	212	2.9 (2.8-3.0)	59.4	19.3
Dec 20-26	10 177	12 792	178	411	2.3 (2.3-2.4)	59.9	55.7
Dec 27–Jan 2	15 934	18 905	276	617	2.2 (2.2-2.3)	60.5	84.7
Jan 3-9	19 219	21 333	331	706	2.1 (2.1-2.2)	60.9	90.1
Jan 10-16	10 185	10 264	174	345	2.0 (1.9-2.0)	61.4	95.6
Jan 17-23	5010	4971	85	169	2.0 (1.9-2.1)	62.0	97.9
Jan 24-30	2581	2575	43	88	2.0 (1.9-2.2)	62.5	99.2
Hospitalizations
5-11 yc
Dec 13-19	0	18	0.00	0.22	∞ (0.3-∞)	4.7	19.2
Dec 20-26	2	50	0.17	0.63	3.6 (1.0-30.9)	10.6	55.7
Dec 27–Jan 2	3	80	0.19	1.04	5.5 (1.8-27.1)	14.4	84.7
Jan 3-9	5	78	0.28	1.04	3.8 (1.6-12.0)	16.5	90.1
Jan 10-16	6	68	0.29	0.94	3.2 (1.4-8.9)	18.5	95.6
Jan 17-23	8	46	0.35	0.65	1.9 (0.9-4.6)	20.9	97.9
Jan 24-30	8	42	0.31	0.60	1.9 (0.9-4.8)	23.4	99.2
12-17 y
Nov 29–Dec 5	2	20	0.04	0.61	16.9 (4.1-148.8)	58.5	0
Dec 6-12	1	11	0.02	0.34	19.0 (2.8-818.3)	58.9	2.1
Dec 13-19	6	23	0.11	0.72	6.8 (2.7-20.4)	59.4	19.3
Dec 20-26	18	45	0.31	1.44	4.6 (2.6-8.4)	59.9	55.7
Dec 27–Jan 2	38	77	0.66	2.50	3.8 (2.5-5.8)	60.5	84.7
Jan 3-9	47	94	0.81	3.10	3.8 (2.7-5.6)	60.9	90.1
Jan 10-16	41	85	0.70	2.84	4.1 (2.8-6.0)	61.4	95.6
Jan 17-23	34	67	0.58	2.26	3.9 (2.6-6.1)	62.0	97.9
Jan 24-30	22	40	0.37	1.36	3.7 (2.1-6.5)	62.5	99.2

^a Partially vaccinated children (anyone with an incomplete primary series or <14 days from primary series completion) in each weekly period were excluded from both the unvaccinated and vaccinated groups.

^b Estimates of the distribution of variants sequenced and reported by the Centers for Disease Control and Prevention National SARS-CoV-2 Strain Surveillance program for US Department of Health and Human Services Region 2, for the earliest date closest to the start of the week (CDC COVID Data Tracker; https://covid.cdc.gov/covid-data-tracker/#variant-proportions).

^c Less than 1% of this age group was fully vaccinated in previous weeks.

For adolescents aged 12 to 17 years, the unvaccinated vs vaccinated IRR against cases declined from 6.7 (95% CI, 6.2-7.2) the week of November 29 to 2.9 (95% CI, 2.8-3.0) the week of December 13 (Omicron 19% sequences), and declined further to 2.0 (95% CI, 1.9-2.2) by January 24 (Omicron >99% sequences) (Table). For unvaccinated vs fully vaccinated children aged 5 to 11 years, the IRR was 3.1 (95% CI, 2.7-3.6) the week of December 13 and declined to 1.1 (95% CI, 1.1-1.2) by January 24. Hospitalizations were higher in unvaccinated than fully vaccinated individuals by the week of January 24, with IRR of 1.9 (95% CI, 0.9-4.8) for children aged 5 to 11 years compared with 3.7 (95% CI, 2.1-6.5) for those aged 12 to 17 years.

For 13 or fewer days after full vaccination, the IRR for unvaccinated vs fully vaccinated children aged 12 to 17 years was 4.3 (95% CI, 3.4-5.3), but by 28 to 34 days it was 2.3 (95% CI, 1.9-2.7) (Figure). For children aged 5 to 11 years, the IRR at 13 days or fewer was 2.9 (95% CI, 2.7-3.1) and at 28 to 34 days it was 1.1 (95% CI, 1.1-1.2).

Figure. New COVID-19 Cases Among Unvaccinated Children vs Fully Vaccinated Children by Time Since Vaccination and Age Group.

Time since full vaccination was defined as days subsequent to 14 days after completion of the primary 2-dose series. Incidence rate ratio (IRR) values less than 1 observed in later times likely reflect estimator instability, residual confounding, or both as opposed to true relative increased risk for those vaccinated.

Discussion

The risks of infection and hospitalization were elevated for unvaccinated vs vaccinated children aged 5 to 11 and 12 to 17 years, although the risk declined as Omicron became more prevalent. Protection declined with time since vaccination. These results complement recent findings of reduced vaccine effectiveness for adolescents against the Delta variant² and the dual effects of the variant and waning protection against infection, with sustained protection against hospitalizations.⁴

Study limitations include that home testing was not reported and could affect case numbers if testing practices differed by vaccination status. Booster doses were not accounted for in the weekly analysis but comprised a small percentage of vaccinations (12.5% of adolescents by January 31, 2022). In the analysis period of time since vaccination, children aged 5 to 11 years were vaccinated soon after vaccine approval; and those aged 12 to 17 years, relatively later. The 2 groups may differ in test seeking or exposures.

These findings support efforts to increase vaccination coverage in children and adolescents and review dosing strategies for children aged 5 to 11 years.⁶

Section Editors: Jody W. Zylke, MD, Deputy Editor; Kristin Walter, MD, Associate Editor.

Supplement.

eMethods.

eFigure. New Cases Among Children Newly Vaccinated From December 13, 2021, to January 2, 2022

eReferences.