Immunogenicity of BNT162b2 in children 6 months to under 5 years of age with previous SARS-CoV-2 infection, in the era of Omicron predominance

Background: Children 6 months to < 5 years old are recommended to receive 3-dose regimen of BNT162b2. Children previously infected with Omicron variant of SARS-CoV-2 develop immunity from natural infection, therefore may require fewer doses of vaccine. Objective: To compare immunogenicity of 1- or 2-dose BNT162b2 in healthy children post COVID-19 with 3-dose BNT162b2 in COVID-na & iuml;ve children. Methods: Children aged 6 months to < 5 years who developed COVID-19 during the Omicron-predominant period were enrolled; Group A 3-6 months(N = 40) and Group B > 6 months(N = 40) prior to vaccination. Participants in Group A and B received 2-dose BNT162b2 intramuscularly 1 month apart. COVID-na & iuml;ve children were enrolled as a control group (N = 40) and received 3-dose BNT162b2 at month 0,1,3. Neutralizing antibody against Omicron variant(BA.2.75 and BA.4/5) was determined by pseudovirus assays(pVNT) as reported by neutralization dilution for 50%inhibition (ID50) at 28 days after the 1(st) and 2(nd) dose. Results: From October-November 2022, 120 children with a median age of 2.8 years (IQR 1.6-4.0) were enrolled. The median duration since COVID-19 to vaccination was 4.4 months(IQR 3.8-5.4) in Group A and 7.9 months(7.0-8.5) in Group B. In Group A, the geometric means(GMs) of pVNT-BA.2.75 ID50 were 553 (95%CI 338-906) and 753(516-1098) after 1 and 2 doses, respectively, and the GMs of pVNT-BA.4/5 ID50 were 1936(1402-2673) and 1885(1414-2512), respectively. In Group B, the GMs of pVNT-BA.2.75 ID50 were 1383(1100-1742) and 1419 (1104-1823), and the GMs of pVNT-BA.4/5 ID50 were 2627(2048-3367) and 2056(1546-2735), respectively. Meanwhile in COVID-na & iuml;ve group, the GMs of pVNT-BA.2.75 and pVNT-BA.4/5 ID50 were 158(98-255) and 59(31-114) after the 3(rd) dose, respectively. The geometric mean ratio(GMR) of pVNT-BA.2.75 ID50 after 1 dose in Group A and B compared with after 3 doses in COVID-na & iuml;ve group were 3.50 (1.93-6.34) and 8.74 (4.79-15.95), respectively. The GMR of pVNT-BA.2.75 ID50 after 1 dose in Group B compared with Group A was 2.50 (1.45-4.31). Conclusions: Children previously infected with SARS-CoV-2 Omicron variant, developed robust neutralizing antibody response against Omicron variant after single-dose BNT162b2. Children with an interval of > 6 months since COVID-19 infection developed higher neutralizing antibody response compared to those with a 3-to-6-month interval.