Data collected from two vaccine safety surveillance programs in the first 10 weeks of administration of third doses of the Pfizer/BioNTech COVID-19 to US children aged 5 to 11 years show that serious adverse events were rare.
A related study in South Korea shows waning vaccine effectiveness (VE) in adolescents after two and three Pfizer doses but sufficient protection against critical illness.
Most side effects mild
In the first study, published today in Morbidity and Mortality Weekly Report, researchers from the Centers for Disease Control and Prevention (CDC) analyzed adverse-event data from the agency's voluntary smart phone-based v-safe vaccine-monitoring program and the Vaccine Adverse Event Reporting System (VAERS) from May 17 to Jul 31, 2022. VAERS is a passive vaccine surveillance system managed by the CDC and the Food and Drug Administration (FDA).
On May 17, the FDA authorized Pfizer COVID-19 boosters for children aged 5 to 11 who had received two doses of the same vaccine at least 5 months earlier.
During the study period, v-safe received information about 3,249 third doses of the Pfizer vaccine in this age-group. The most common adverse reactions included injection-site pain (66.7%), fatigue (28.9%), and headache (19.9%). Most reactions were reported the day after vaccine receipt and were mild.
After a third dose, injection-site and systemic reactions were often reported (68.5% and 45.6%, respectively). Injection-site and systemic reactions had been documented in similar proportions after the second dose (68.0% and 45.8%, respectively).
While most adverse effects reported by those who experienced pain, fatigue, headache, or muscle pain were mild, more moderate and severe symptoms were reported after the third dose than after the second.
During the week after the third dose, 6.9% of children didn't attend school because of adverse effects (vs 10.0% after dose 2), and 12.1% couldn't participate in daily activities (vs 7.5% after dose 2). About 1.0% of parents said they sought medical attention for their child after the third dose (vs 0.9% after dose 2), most often at a clinic (0.5%) or through telemedicine (0.3%). No children were hospitalized.
Over the study period, VAERS received 581 reports of one or more adverse effects after third-dose COVID-19 vaccination. Nearly all (99.5%) of reports were considered not serious. The most common nonserious events (71%) were related to vaccine preparation or administration errors (eg, incorrect dose, inappropriate patient age), and 15.3% of the 413 reports also included an adverse health event.
Other frequently reported events included fever (7.8%), limb pain (6.6%), and fatigue (4.8%). There were three reports of serious events: new-onset type 1 diabetes 10 days after vaccine receipt, facial swelling 3 days after receipt, and generalized pain, fatigue, and malaise after 5 days, the latter of which required hospitalization. There were no reports of myocarditis (inflammation of the heart muscle) or death.
Waning Pfizer vaccine efficacy in teens
In a related study published yesterday in JAMA Network Open, researchers compared the rates of COVID-19 infection and critical infection by age, region, vaccination status, and number of Pfizer COVID-19 vaccine doses in all adolescents aged 12 to 18 years in South Korea from Jul 19, 2021, to Jan 22, 2022. The Omicron variant emerged in late November 2021.
Critical illness was considered hospitalization with high-flow oxygen, mechanical ventilation, extracorporeal membrane oxygenation (ECMO), or kidney dialysis within 28 days of COVID-19 diagnosis.
Among the 3,203,985 adolescents, 57.4% were 12 to 15 years old, 27.6% were 16 or 17, and 15.0% were 18. Of them, 29,285 (0.9%) had COVID-19, and 11 (0.0003%) had a critical infection. Of the infected adolescents, 55.4% were unvaccinated, while 17.3% and 24.0% had received one and two doses, respectively. None of the 11 critically ill adolescents were vaccinated.
Estimated vaccine effectiveness (VE) 0 to 13 days after two doses was 75.5% among 18-year-olds, 80.4% among those aged 16 or 17, and 79.2% among those 12 to 15. Thirty to 59 days after a second dose, VE waned to 71.3% among 18-year-olds, 61.5% among those 16 or 17, and 49.6% among those 12 to 15. Thirty to 59 days after a third dose, VE fell to 55.2% among 18-year-olds (data on younger adolescents were not available).
"Our results suggest that after vaccination with BNT162b2 [Pfizer], the effectiveness of the vaccine against SARS-CoV-2 infection among adolescents waned over time, with limited protection observed 30 to 59 days after administration of a second dose of the vaccine," the study authors wrote. "Regardless, our findings suggest an association between the BNT162b2 vaccine and sustained effectiveness against critical SARS-CoV-2 infection, as reported previously."
Two Pfizer vaccine doses, they added, "may be sufficient for protection against critical SARS-CoV-2 infection among adolescents, albeit with waning immunity. Furthermore, booster vaccinations may be required for continued protection among adolescents."