A study published on a preprint server describes the kinetics of antibody response to the earliest approved vaccines against COVID-19, which were developed on an mRNA platform. Humoral responses occur in the form of multiple antibody subsets, of which both IgG and IgA are key to neutralization of the virus, but have different parts to play in the immune response, occurring at different stages of infection and different sites in the host organism.
The current study aimed to evaluate the titers of IgA and IgG in serum against the SARS-CoV-2 spike antigen in the earliest vaccine recipients. These four individuals were healthcare workers and thus at high priority for the vaccine. The antibody levels in these workers were measured for a maximum of 80 days from the first dose of the vaccine. The baseline tests for SARS-CoV-2 nucleocapsid (N) and spike (S) antigens were negative.
After the first dose, the serum levels of the spike-directed IgG showed an exponential increase, before eventually evening out at 18-21 days. A similar rise occurred after the second dose, to reach the peak at seven days from the vaccination. Over the rest of the follow-up period, approximately 20-50 days, the IgG values plateaued at about 80% of peak values. Spike-specific IgA levels showed a similar trend, peaking within the same period as IgG after both first and second doses of the vaccine.
However, the drop in titer with IgA after the maximum titer was significantly more rapid than that with IgG. IgA antibody levels thus dropped to about half the titer at the peak response, following the first dose. After the booster dose, it peaked and then plateaued at about 40% of the maximal dose, within 50 days of the second injection.