Conclusions
Recent studies show that even if the vaccine is very effective in reducing severe forms of COVID-19 in nursing home residents, the vaccination, even complete, may not totally prevent the risk of SARS-CoV-2 outbreaks in nursing homes (Blain et al., submitted).41
This present study supports the hypothesis that i ) most of the residents having recovered from COVID-19 (with a diagnosis using RT-PCR or N-protein IgG) may only require one single vaccine dose to achieve peak antibody and memory B cell responses. ii ) However, the second dose reduces the prevalence of residents with low S-protein IgG levels (≤1050 AU/mL)(from 11.5% to 2.7%). iii ) For individuals whose infection history is unknown, measuring S-protein IgG antibody levels just before the second jab could be useful in determining whether a second dose is required. iv ) Most of the individuals without prior COVID-19 have a low level of S-protein IgG 3 weeks after the first jab, further confirming the benefit to administer the second dose without delay to boost the antibody response. After two doses, around 30% of residents without prior COVID-19 have a low level of S-protein IgG. Whether these residents with low antibody response after the complete vaccine regimen are at higher risk of symptomatic COVID-19 and whether they may benefit from a third vaccine dose remains to be further investigated.
Additional studies are required to demonstrate whether the measure of S-protein IgG after the vaccination should help to personalise the vaccine schedules and reduce secondary effects related to possible reactogenicity 42. Further studies are necessary also to determine whether the present results obtained in residents infected by the natural strain of the virus may be replicated in residents infected by the new variants 43,44.