Discussion
Recent studies conducted in small samples of immunocompetent adults showed higher levels of S-protein IgG after a single jab in individuals with prior COVID-19 than in those without prior COVID-19.5,23 Our pilot study with 136 residents was the only one to include NH residents. 16 The current study is innovative because it includes a large sample of NH residents and evaluates the effect on antibody response of (i) the time elapsed since COVID-19 infection and vaccination, and (ii) the persistence of a natural immunity against SARS-CoV-2 N-protein at the time of the first vaccination. The large sample allows the assessment of the percentage of residents with undetectable S-protein IgG (< 50 AU/mL) or with low levels of S-protein (≤1050 AU/mL) after two vaccine doses among those with or without prior COVID-19. It also enables the comparison of antibody response after one jab in residents without COVID-19 and after two doses in residents with prior COVID-19.
The present study shows that 3 weeks after the first vaccine dose:i ) 41.7% of residents without prior COVID-19 have undetectable S-protein IgG and 91.3% have low levels of S-protein IgG, ii ) only 3.8% of residents without prior COVID-19 have undetectable S-protein IgG and 11.5% have low levels of S-protein IgG, iii ) among residents with prior COVID-19, those seropositive for N-protein at the time of the vaccine and those with an older infection (in the last 9 to 12 months) have the highest S-protein IgG levels, iv ) the second jab significantly boosts the antibody response of residents without prior COVID-19 (since 6 weeks after the second dose, only 3.1% remain seronegative for S-protein IgG). However, v ) 29.4% of residents without prior COVID-19 have low S-protein IgG levels,vi ) in residents with prior COVID-19, the second dose leads to a reduced percentage of residents with low S-protein IgG levels (from 11.5% after the first dose to 2.7% after the second dose). The median value of S-protein IgG levels after one jab in residents with prior COVID-19 exceeds that of two doses in residents without prior COVID-19.