A single dose of the Pfizer vaccine reduces the risk of Covid-19 infection in children, new research has found.

If infected with the Delta or Omicron variants, children vaccinated with one dose of Pfizer were found to have a milder case of Covid-19 and a reduced risk of re-infection for at least 90 days.

The vaccine was well tolerated in children, and the benefits were seen even if children had previously been infected with Covid-19, the study found.

The results were published in the Lancet Regional Health by scientists from King’s College London and ZOE Ltd.

The researchers examined data from 115,775 children aged 12-17 years which was proxy-reported through the ZOE COVID Symptom Study app. Data was collected after August 5, 2021, when the UK Government introduced universal vaccination for people under 18, until February 2022. For much of this period, children could only access a single dose of the Pfizer vaccine, and the study was limited to assessing responses to a single dose.

Currently, clinically vulnerable children aged 5-11 years and all 12-17 years can access two doses, with young people aged 16-17 able to get an additional booster dose.

Professor Emma Duncan 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.’

The researchers found a significant and rapid reduction in infection risk after a single dose of vaccine. At 14-30 days, vaccinated children were 80% less likely to catch the Delta variant and 54% less likely to catch the Omicron variant than unvaccinated children. At three months post-infection vaccinated children were 62% and 64% less likely to catch Delta and Omicron variants, respectively.

Covid-19 symptoms were mild in all children, whether vaccinated or not. But the researchers found that vaccinated children infected with the Delta variant, generally had milder symptoms of Covid-19, with a reduction in multiple symptoms in younger children and a reduced chance of specific symptoms such as fatigue, sore throat and fever in older children.

The Omicron variant produced a milder disease in both vaccinated and unvaccinated children than the Delta variant. However, the researchers found several Omicron symptoms were less common in younger vaccinated children, such as abdominal and muscle pain, loss of smell, fever and low appetite.

Dr Michael Absoud, a consultant paediatrician and senior author of the study, 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.’

This comes after it emerged that a new Omicron Covid vaccine could be used for autumn boosters in the UK if approved by the MHRA, after Moderna said it has ‘completed regulatory submissions’ for the jab.