INTRODUCTION: The COVID-19 pandemic has posed serious challenges for healthcare. Immunization against SARS-CoV-2 was initially effective, but the immune response gradually waned as time passed. As protection declined and new virus variants emerged, the incidence of virus infection increased even among those who had been vaccinated three times. In this study, we compared the immune response to booster vaccination and subsequent SARS-CoV-2 infection in previously uninfected individuals.

METHODS: Humoral immune responses were monitored by an IgG serological assay detecting neutralizing antibodies. Cellular immune responses were monitored by a SARS-CoV-2 specific interferon-gamma release assay.

RESULTS: A significant proportion of the participants in our previous booster vaccination study were subsequently infected with SARS-CoV-2. The immune response of 16 of these volunteers was followed up over several months.  IgG levels peaked after approximately 1 month in both conditions, but at significantly higher levels upon viral infection than following vaccination (26.1 and 15.0 S/CO, respectively).

Circulating IgG levels 4 months after booster vaccination were only 39% of the 1-month maximum. After viral infection, the corresponding value was 78%. Similar differences were observed at the cellular level, but there was no correlation between humoral and cellular immune responses.

CONCLUSION: Compared to the effects of vaccination, SARS-CoV-2 infection elicits a more pronounced immune response, both cellular and humoral, which declines significantly more slowly with time.