​Children aged five to 11 who had received two doses of the vaccine before catching Covid-19 were 85.3 per cent less likely to be infected by Omicron sub-variants BA.4 and BA.5. PHOTO: ST FILE

The Pfizer-BioNTech vaccine works best in protecting children from contracting the Omicron sub-variants BA.4 and BA.5 if they get inoculated before their first Covid-19 infection, a study has found.

Children aged five to 11 who received two doses of the vaccine prior to catching Covid-19 were 85.3 per cent less likely to be infected by the two sub-variants, said the study led by KK Women's and Children's Hospital (KKH).

In comparison, children in this age group who received two vaccine doses after contracting Covid-19 were 74 per cent less likely to be infected by the sub-variants.

KKH said further research is needed to explain the observed trend of children getting better protection if they are vaccinated before infection.

The study published in May was done based on nearly 150,000 children and youth aged five to 17 who were infected with Covid-19 between January 2020 and December 2022. It studied the effects of only the Pfizer-BioNTech vaccine.

Omicron infections were the most common during the period of the study, which was done by KKH, the National Centre for Infectious Diseases and the Ministry of Health.

Lead author and KKH senior consultant Yung Chee Fu said: "Our study found that getting vaccinated has added protection against Covid-19 variants for children and adolescents who had recovered from a previous infection.

"In children, the best protection was achieved among those who got vaccinated before their first infection."

Associate Professor Yung, who specialises in paediatric infectious diseases, said the finding reiterates the point that parents should not rely on past Covid-19 infections alone for their children's immunity against Omicron variants.

Those aged 12 to 17 who caught Covid-19 before getting vaccinated were 85.7 per cent less likely to be infected by Omicron BA.4 and BA.5.

They had a higher level of immunity than those who were vaccinated before infection (82.9 per cent less likely to be infected).

The Pfizer-BioNTech vaccine was also found to confer a lower level of immunity against the Omicron XBB sub-variant compared with BA.4 and BA.5. The study said this was expected as significant molecular changes in XBB made it more transmissible.

The type of Covid-19 variant an individual was previously infected with had an impact on how much protection the vaccine conferred. Those who were infected with the Delta variant were found to have the lowest level of protection against subsequent BA.4 and BA.5 infections.

Prof Yung said the study is directly relevant to the Singapore population as many have already caught Covid-19.

"For unvaccinated individuals who have recovered from Covid-19, vaccination will provide added protection, and they can get vaccinated three months after their infection," he added.