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A single dose of the Pfizer vaccine yielded a significant and rapid reduction in the risk of COVID-19 infection in children during the Delta variant period, a preprint study has found from Researchers at King’s College London and ZOE Ltd. However the protective effect was variant-dependent and the benefits “were noticeably less pronounced during the period of Omicron”, the researchers said. Moreover both vaccinated and unvaccinated children generally 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 made available to non-vulnerable children for the first time, and February 14, 2022. Researchers assessed proxy-reported symptoms for 115,775 children and young people aged 12-17 years, of whom 25,971 received one dose of BNT162b2 vaccine.
They found “a significant and rapid reduction in infection risk” after a single dose of vaccine while 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, figures were -53.7%, -57.9%, and -63.7%, respectively. Protection waned with time, particularly during Omicron.
The results follow those of a recent study from New York State showing rapid loss of protection against infection in children in the Omicron variant era. Vaccine efficacy plummeted 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% towards the end of January 2022. In the 5- to 11-year-old age group (not included in the UK study), the New York results showed vaccine efficacy decreased from 68% in the week of December 13, to 12% by January 24.
The UK study showed that the reduction in risk was greater in children who had had prior SARS-CoV-2 infection. Vaccinated children who had previously reported a positive test for SARS-CoV-2 maintained a reinfection risk close to zero until at least 100 days while Delta remained the main variant. The authors could not make a similar comment for Omicron due to lack of data.
Infection Mild Even in Unvaccinated Children
During the Delta period, the single dose was associated with milder infection symptoms, which were also of shorter duration, although unvaccinated individuals generally had an uncomplicated course too, the researchers said.
During the Omicron period, number of symptoms and duration of illness was generally milder than Delta for both vaccinated and unvaccinated children. “Very few children in the study population presented to hospital during either Delta or Omicron periods.”
Both vaccinated and unvaccinated children aged 16-17 had similar disease burden, without any significant differences in odds for any individual symptom. However during the Delta period, several symptoms were less common in younger vaccinated children, including headache, dizziness, and eye soreness.
The results again showed that COVID-19 was usually mild in unvaccinated children, though vaccinated children who contracted COVID-19 may have had an even milder course, the researchers said.
Senior author Professor Emma Duncan, PhD, from King’s College London, said: “Our paper will provide useful information for parents 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 get COVID-19, and if infected post-vaccination they are likely to have a milder disease course – at least for Delta and Omicron variants. Our paper is also reassuring that Omicron in children is usually mild, even in unvaccinated children.”
In terms of adverse reactions, data showed that side-effects at the injection site were common following vaccination, with arm tenderness and local pain the most reported symptoms. Systemic side-effects, such as headache 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 symptoms post-vaccination settled down quickly.
The research was published as a preprint study on MedRxiv and has not been peer-reviewed. For much of the period covered by the data, only one dose of the Pfizer vaccine was offered for children who were not clinically vulnerable. Currently, clinically vulnerable children aged 5-11 years and all those aged 12-17 years can get two doses, with those aged 16-17 also eligible for an additional (booster) dose.
Dr Michael Absoud, a senior author of the study and consultant and reader at King’s College London, said: “Our paper has implications for future policies pertaining to health resource allocation and prioritisation. Overall, a single dose of vaccination reduces COVID-19 symptoms but offers time-limited protection against infection, especially for Omicron. Our study also shows that infection-acquired immunity is additionally protective. It is reassuring that children recover well post-SARS-CoV-2 infection with Delta and Omicron.”
Créditos: Comité científico Covid