Among 12- to 18-year-old hospitalized COVID-19 patients, two doses of the Pfizer/BioNTech vaccine was 91% effective in preventing the rare but serious coronavirus-related multisystem inflammatory syndrome in children (MIS-C), according to a US study published late last week in Morbidity and Mortality Weekly Report.
In the first real-world evaluation of the Pfizer vaccine's effectiveness against MIS-C, a team led by Centers for Disease Control and Prevention (CDC) researchers compared the odds of COVID-19 vaccination in 102 adolescent MIS-C patients and 181 controls at 24 sites.
Controls included 90 COVID-19–negative patients and 91 MIS-C–negative patients from Jul 1 to Dec 9, 2021, after the emergence of the more virulent Delta (B1617.2) variant but before the even more highly transmissible Omicron (B.1.1.529) strain.
Median participant age was 14.5 years, and 58% had one or more underlying medical conditions. About 36% of controls and 5% of COVID-19 patients were fully vaccinated at least 28 days before hospitalization.
MIS-C, which usually occurs 2 to 6 weeks after a mild or asymptomatic COVID-19 infection, is a hyperinflammatory disorder marked by severe illness requiring hospitalization and involvement of at least two organ systems.
Critical MIS-C cases only in unvaccinated
Estimated vaccine effectiveness against MIS-C was 91% (95% confidence interval [CI], 78% to 97%). A sensitivity analysis that excluded patients with a positive COVID-19 or serologic test estimated vaccine effectiveness at 90% (95% CI, 75% to 96%).
Regardless of control group, effectiveness against MIS-C was similar, at 92% in COVID 19–negative controls (95% CI, 77% to 97%) and 89% in controls without MIS-C (95% CI, 70% to 96%).
Sixty-one percent of case-patients were admitted to an intensive care unit (ICU), and 37% required respiratory or cardiovascular life support. All 38 MIS-C patients requiring life support were unvaccinated, including 9 patients requiring invasive mechanical ventilation, 35 needing vasoactive drugs, and 1 needing extracorporeal membrane oxygenation.
The length of hospital stays between vaccinated and unvaccinated participants were similar, at a median of 5 days, and no deaths were reported. Overall, 89% of case-patients had cardiovascular system involvement, 82% had gastrointestinal involvement, and 67% had hematologic (blood) involvement.
The researchers cautioned that the time point at which two doses of COVID-19 vaccine protect against MIS-C is unknown and that while the participating hospitals covered a broad geographic area, the findings are not generalizable to the rest of the US pediatric population. Regardless, they said, the results support COVID-19 vaccination in this age-group.
"This analysis lends supportive evidence that vaccination of children and adolescents is highly protective against MIS-C and COVID-19 and underscores the importance of vaccination of all eligible children," the authors concluded.