Vax Protection Against Omicron More Durable for Adolescents Than Young Kids

 Protection with mRNA vaccination for COVID-19 faded rapidly against infections from the Omicron variant in younger children, though proved more durable in adolescents, likely due to the higher dose, real-world data from Qatar suggested.

In kids ages 5-11, vaccine effectiveness with a primary series of Pfizer's vaccine (Comirnaty) was 25.7% (95% CI 10.0-38.6), falling from 49.6% immediately after the second 10-μg dose to a negligible and non-significant 11% after 3 months, according to Hiam Chemaitelly, PhD, of Weill Cornell Medicine-Qatar in Doha, and colleagues.

The number needed to vaccinate in order to prevent one documented Omicron infection was 333 in this group, they wrote in the New England Journal of Medicine.

"Currently approved vaccines for children 5-11 years provide protection against infection," Chemaitelly told MedPage Today, but this protection "was no match" for Omicron's immune evasion.

In adolescents ages 12-17, the 30-μg primary vaccine series -- the same dose of Pfizer's vaccine given to adults -- yielded an overall vaccine effectiveness of 30.6% (95% CI 26.9-34.1), but many in this group had received their vaccine months earlier.

Vaccine effectiveness was highest, at 51.3%, for adolescents who had most recently received their second dose, and 35.5% for the those who received their vaccinations during the time frame when the vaccine was rolled out for the younger group of kids, the researchers reported.

The number needed to treat to prevent one infection among those ages 12-17 was 30.3 before Omicron and 20.8 during Omicron.

"The 30-μg dose in adolescents was associated with stronger and more durable protection, a finding that suggests a role for the antigen dose in determining protection," wrote Chemaitelly and co-authors.

"Our findings suggest the need to reconsider the value and strategies of vaccinating healthy children in the Omicron era with the use of currently available vaccines," the group concluded.

Chemaitelly and colleagues compiled data from three matched retrospective cohort studies from Qatar that included 18,728 children ages 5-11 vaccinated with the approved two-dose (10 μg) primary series from February to July 2022 (Omicron period); 23,317 adolescents ages 12-17 vaccinated with the approved two-dose (30 μg) series for this age group during a pre-Omicron period (February to November 2021); and finally 17,903 adolescents vaccinated during a period that included Omicron (February 2021 to July 2022).

Each were matched 1:1 with an unvaccinated control group based on their demographics, and number of coexisting conditions. Vaccinated participants were also matched by the month of their second dose with a negative COVID test result in the unvaccinated controls. Effectiveness of the vaccine was measured starting from 14 days after the second vaccine dose. Children with a previous documented SARS-CoV-2 infection were excluded.

Cumulative incidence of infection in children ages 5-11 was lower among the vaccinated subset (2.1% vs 2.4% among controls at 110 days), as was the case for adolescents in the pre-Omicron period (0.8% vs 4.1%, respectively, at 135 days) and the longer period that included Omicron (15.9% vs 20.7% at 195 days).

In the 5-11 group, vaccine effectiveness against Omicron was stronger in the youngest kids :

• 5-7 years: 46.3% (95% CI 21.5-63.3)
• 8-11 years: 16.6% (95% CI -4.2 to 33.2)

And a similar pattern played out for adolescents ages 12 to 17 during Omicron:

• 12-14 years: 35.6% (95% CI 31.2-39.6)
• 15-17 years: 20.9% (95% CI 13.8-27.4)

Vaccine effectiveness for adolescents was highest in the pre-Omicron cohort, at 87.6% (95% CI 84.0-90.4), and the protection here waned slowly.

"Protection was approximately 95% right after receipt of the second dose and remained strong at more than 50% for at least 5 months," wrote Chemaitelly and colleagues. "Protection and waning patterns that were associated with the 30-μg dose among adolescents paralleled those among adults, although protection seemed to be slightly stronger among adolescents than among adults."

Across the three cohorts, the incidence of infections that progressed to severe, critical, or deadly COVID-19 were rare, including none in the 5-11 group; none in the pre-Omicron 12-17 group; and three cases in the Omicron adolescent group (one critical case among the vaccinated and unvaccinated subsets, and one severe case in the unvaccinated group).

The authors acknowledged limitations to the data, including potential unmeasured confounding. Testing frequency and guidelines differed among cohorts, though sensitivity analyses adjusting for these differences still found similar results. Also, since only a few infections were indicative of severe COVID-19, they could not adequately estimate vaccinate effectiveness against severe disease.

Disclosures

This study was supported by the Biomedical Research Program as well as the Biostatistics, Epidemiology, and Biomathematics Research Core at Weill Cornell Medicine–Qatar, Hamad Medical Corporation, the Ministry of Public Health in Qatar, and Sidra Medicine. Reagents for the viral genome sequencing were provided by the Qatar Genome Program and Qatar University Biomedical Research Center.

Chemaitelly and coauthors disclosed no competing interests.

Primary Source

New England Journal of Medicine

Source Reference: Chemaitelly H, et al "Covid-19 vaccine protection among children and adolescents in Qatar" N Engl J Med 2022; DOI: 10.1056/NEJMoa2210058.

https://www.medpagetoday.com/infectiousdisease/covid19vaccine/101542