ABSTRACT
Background : Various studies have pinned longevity of protective Immunoglobulin-G (IgG) titres at 2-5 months. The robustness and longevity of the IgG antibody response to COVID-19 infection has been gauged in a cohort of 214 single institutional health care workers by serial quantitative immunometric tests. Currently no separate guidelines exist for vaccination of COVID-survivors and this study provides data to fill this lacuna in knowledge. Methodology : Prospective longitudinal panel survey administered to the same cohort of Health Care Workers (HCW) till such time they got vaccinated under Indian Government’s free vaccination drive for HCW. Depending upon the date of contraction of infection the HCW could be longitudinally monitored for variable periods (2-9 months). The survey questionnaire comprising multiple-choice, dichotomous, matrix and Likert-scale questions was deployed to the respondents online via email/WhatsApp. Data was expressed as box-whisker plots, trendlines and trend areas. A p-value<0.05 was considered statistically significant. The composite index of ‘Effective Immunity’ was calculated. Results : The mean IgG antibody titre was 11.13±8.6AU at 1-2m, 9.68±8.9AU at 3-4m, 8.35±5.9 AU at 6-7m and 7.87±4.4 AU at 8-9m after first symptom, respectively. The lowest titre at all time points was 0 while the highest titres were 46.8 AU, 56.5 AU, 23.4 AU and 17.4 AU at 1-2m, 3-4m, 6-7m and 8-9m, respectively. Conclusion : Adaptive active immunity acquired through natural infection may last for at least 9 months post-initial exposure and lies in the moderate protection range in 77% HCW, which can be extrapolated to vaccination and immunity passports. Separate vaccination guidelines are required for COVID-survivors. The first shot of vaccine serves as a booster second exposure/booster dose in all COVID-survivors.HCW with low IgG-titre may suffer from a false sense of security. Periodic quantitative IgG-titre based serological tests can help guide timing of second shot of vaccination and predict likelihood of re-infection