A single dose of the Pfizer vaccine resulted in a significant and rapid reduction in the risk of COVID-19 infection in children during the Delta variant period, according to a pre-print study by researchers from King’s College London and ZOE Ltd. However, the protective effect was variable. -dependent and benefits “were significantly less pronounced during the Omicron period,” the researchers said. Additionally, vaccinated and unvaccinated children usually had only mild infection.
The prospective longitudinal cohort study used data from the ZOE symptom app between August 5, 2021, the day after the vaccine was first made available to non-vulnerable children, and February 14, 2022. The researchers assessed proxy-reported symptoms for 115,775 children and youth aged 12-17 years, of whom 25,971 received one dose of BNT162b2 vaccine.
They found “a significant and rapid reduction in risk of infection” after a single dose of vaccine when the Delta variant was dominant, reaching -80.4% at 14-30 days, -86.4% at 31-60 days and -61.5% after 61-90 days. For the Omicron variant, the numbers were -53.7%, -57.9%, and -63.7%, respectively. Protection decreased over time, especially during Omicron.
The results follow those of a recent study from New York State showing a rapid loss of protection against infection in children in the era of the Omicron variant. Vaccine efficacy dropped in children aged 12-17 once Omicron took hold and accounted for 99% of circulating virus strains, with efficacy falling from 85% at the end of November 2021 to 51% at the end of January 2022. In the 11-year-old age group (not included in the UK study), the New York results showed that vaccine effectiveness fell from 68% the week of December 13 to 12% the week of December 13. January 24.
The UK study showed that the risk reduction was greater in children who had previously been infected with SARS-CoV-2. Vaccinated children who previously reported testing positive for SARS-CoV-2 maintained near-zero risk of reinfection until at least 100 days, while Delta remained the primary variant. The authors could not make a similar comment for Omicron due to lack of data.
Mild infection even in unvaccinated children
During the Delta period, the single dose was associated with milder symptoms of infection, which were also of shorter duration, although those not vaccinated generally had an uncomplicated course as well, the researchers said.
During the Omicron period, the number of symptoms and the duration of illness were generally milder than Delta for vaccinated and unvaccinated children. “Very few children in the study population presented to the hospital during the Delta or Omicron periods.”
Vaccinated and unvaccinated children aged 16 to 17 had a similar disease burden, with no significant difference in ratings for any individual symptom. However, during the Delta period, several symptoms were less common in young vaccinated children, including headaches, dizziness and eye pain.
The results again showed that COVID-19 was generally mild in unvaccinated children, although vaccinated children who contracted COVID-19 may have had an even milder course, the researchers said.
Lead author Professor Emma Duncan, PhD, of King’s College London, said: “Our article will provide useful information for parents who are considering vaccinating their children against SARS-CoV-2. Even a single dose of the Pfizer vaccine means children and young people are less likely to contract COVID-19, and if infected after vaccination they are likely to have a milder course of the disease – at least for Delta and Omicron variants. Our article also reassures that Omicron in children is generally benign, even in unvaccinated children.
In terms of adverse effects, the data showed that injection site side effects were common after vaccination, with arm tenderness and local pain being the most reported symptoms. Systemic side effects, such as headaches and fatigue, were less common, but affected 8.3% of 16 to 17 year olds and 14.1% of 12 to 15 year olds. In almost all cases, post-vaccination symptoms quickly subsided.
The research was published as a preprint study on MedRxiv and has not been peer-reviewed. For much of the time period covered by the data, a single dose of the Pfizer vaccine was offered to children who were not clinically vulnerable. Currently, clinically vulnerable children aged 5 to 11 and all those aged 12 to 17 can receive two doses, with those aged 16 to 17 also eligible for an additional (booster) dose.
Dr Michael Absoud, lead author of the study and consultant and reader at King’s College London, said: “Our paper has implications for future policy relating to the allocation and prioritization of health resources. “Together, a single dose of vaccination reduces symptoms of COVID-19 but provides time-limited protection against infection, especially for Omicron. Our study also shows that acquired immunity against infection is additionally protective It is reassuring that children recover well after SARS-CoV-2 infection with Delta and Omicron.
The study was supported by the Wellcome Foundation and the National Institute for Health Research.