Comparative analysis of post-vaccination anti-spike IgG antibodies in old Nursing Home Residents and in middle-aged Healthcare workers

The current consensus is that 2 doses of mRNA vaccines against SARS-CoV-2 are needed for people without COVID-19 history, while for those who had suffered COVID-19, a single dose may be enough to achieve high levels of immunization. This consensus has been based on results obtained in middle-aged populations, whereas only few data exist for the oldest and most frail adults such as nursing home residents (NH-Res) that is, the population most vulnerable to develop severe forms of COVID-19. In this study, we studied the anti-SARS-CoV-2 IgG(S) level of NH-Res and healthcare workers (HCWs) with or without a history of COVID-19 infection taking into account the time since immunization (COVID-19 and/or vaccination). 654 subjects were analyzed: 397 NH-Res (median age 88, IQR 82-93 years, 75% women) and 257 Health Care Workers (HCWs, median 46, IQR 38-54 years, 81% women). NH-Res and HCWs were classified in one of the following 3 groups: No-COVID history and 2 vaccine shots (COV-NO/2VACC); Yes-COVID history and 1 (COV-YES/1VACC) or 2 (COV-YES/2VACC) vaccine injections. The time-related decrease in IgG (S) in subjects without COVID-19 history, SARS-COV-2 serology would be negative in HCWs approximately 220 days and in residents 180 days after vaccination. This time-related decrease was much slower in those with history of COVID. NH-Res belonging into the COV-NO/2VACC and the COV-YES/1VACC groups showed lower IgG (S) levels than the same groups of HCWs (for both groups, p<0.0001), whereas in the group COV-YES/2VACC, IgG (S) levels were similar in NH-Res and HCWs (p=0.88). These results remained unchanged after adjustment for age and duration since immunization. Thus, in NH-Res, 2 vaccine shots were associated with a more pronounced immune response, whereas in HCWs, 1 or 2 vaccine shots in patients with COVID-19 history did make any difference. These results indicate significant differences in mRNA vaccination between NH-Res and middle-aged controls, and could contribute to the specification of vaccine policy in this very old, frail population.