Additional evidence of natural boosting in immunity for varicella: A case study in nursery school children born during or after the COVID-19 pandemic in Japan

An earlier study proved natural boosting among young adults against community outbreaks, as measured by antibody titer for varicella, rubella, and mumps. Natural boosting of varicella immunity during the pandemic was measured. After collecting varicella vaccination dates and infection information for children of 4 cooperating nursery schools through the end of March 2025, we regressed the daily varicella infection status of each child on the number of weeks after their second vaccination and on the outbreak situation in areas surrounding the nursery schools, as reported by National Official Sentinel Surveillance for Infectious Diseases, with a lag. Immunization and infection records of 359 children of the 4 nursery schools were received. Of those children, 297 had received 2 doses of vaccine for varicella. Logistic regression revealed that attenuation, the period after the second vaccination was positively associated, and natural boosting, community outbreak with four-weeks lag was negatively associated with infection. Direct effect, effect of community outbreak with two-weeks lag to infection, was not confirmed. Results suggested natural boosting of immunity for varicella, even in young children. However, because of data limitations, these findings cannot be compared to data obtained before the pandemic. • We considered varicella infection after vaccination in nursey school children. • Infection negatively associated with community outbreaks with a 5–6 week lags. • It suggests natural boosting in varicella among young children. • Infection did not positively associate with community outbreaks with 0–2 week lag. • Duration after second vaccination was positively associated with infection.