Active immunization by COVID-19 mRNA vaccine results in rapid antibody response and virus reduction in breakthrough infection by Delta (B.1.617.2)

Vaccination is expected to suppress COVID-19 infection. However, breakthrough infections have increased following vaccination because of the spread of variants of concern, notably Delta (B.1.617.2 lineage). Virological and serological data pertaining to post-vaccination infections are limited. Here, we conducted genome analysis determined the viral lineages that infected patients following vaccination. Changes in viral load, antibody levels, and viral antigen levels following infection were analyzed. At the time of infection, Delta-infected patients had a 6.2-fold and 12.3-fold higher viral load compared with Alpha and other lineages, respectively. Viral lineages (Delta:Alpha:Other) of infection were 0:12:0 in the fully vaccinated group, 1:11:0 in the partially vaccinated group, 9:16:0 in the shortly after vaccination group, and 254:229:165 in the unvaccinated group. Breakthrough infections occurred regardless of retention of high antibody titers following vaccination. At the time of diagnosis, Delta-infected patients showed high viral load with or without vaccination. However, no fully vaccinated patients developed severe disease, and the rapid increase in anti-spike antibodies occurred approximately 1 week after onset of symptoms. Concomitantly, a decrease in viral antigen levels was observed in fully vaccinated patients, shortening the time to negative result by approximately 2 days compared with unvaccinated patients. Collectively, even if breakthrough infection occurs, the rapid antibody response in fully vaccinated individuals contributes to prevention of severe disease, possibly because of suppression of viral replication.