According to results from the UK government′s SIREN (SARS-CoV-2 Immunity and Reinfection Evaluation) trial, a longer gap between the first and second doses of the Pfizer/BioNTech (BNT162b2) vaccine resulted in a ninefold increase in COVID-19 antibody levels in patients who had never been infected.

The antibody level was interval-dependent and varied by age, with those aged 45-54 years exhibiting an 11-fold rise with a gap of more than 10 weeks compared to 2-4 weeks. People under the age of 25 exhibited a 13-fold increase with the long gap, but this sub-group had a small number of participants. The antibody levels were 1970.83 (1506.01 - 2579.1) BAU/mL with a 3-month interval compared to 13,759.31 (8,097.78 - 23,379.09) BAU/mL after a 9-month interval.

The researchers evaluated the anti-spike binding antibodies in serum samples from 5871 healthcare personnel, with 3989 receiving one dosage (at least 21 days) and 1882 receiving two doses (at least 14 days). Participants were also divided into two groups: those who had evidence of spontaneous COVID-19 infection and those who had not been exposed to the virus. After immunization, almost all (>99 per cent) of individuals were infection-naive sero converted.

The overall antibody levels in infection-naive participants ranged from 1268.72 Binding Antibody Units (BAU)/mL (1043.25 - 1542.91) to 11,479.73 BAU/mL (10,742.78 - 12,267.24) (P.0001) among those with a 2-4-week interval compared to those with a 10-week delay respectively. After the first vaccine dosage, the mean antibody (anti-S titer) in the infection-naive group was 75.48 BAU/mL (antibody binding unit/mL), which increased to 7049.76 BAU/mL after the second dose, while antibody titers increased from 2111.08 BAU/mL after the first dosage and 16,052.39 BAU/mL after the second dose in the post-infection group. (Medscape - Apr 24, 2022)