A study published in the Journal Science Immunology revealed that people previously exposed to SARS-CoV-2 produced a more robust immune response after receiving the first but not the second dose of the mRNA vaccine. The researchers began recruiting participants from the University of Pennsylvania Health System in December 2020, with 44 individuals enrolled at the time of publication. About 33 participants reported no previous SARS-CoV-2 infection, and 11 reported prior exposure ranging from 65 to 275 days before vaccination.
Everyone received a Pfizer-BioNTech or Moderna mRNA vaccine. To investigate changes in humoral immunity, the researchers collected blood samples at the start of the trial, 2 weeks after the first shot, the day of the second shot, and 1 week after the second mRNA dose. After the first mRNA dose, both groups produced antibodies, and recovered individuals showed a more tremendous boost in their SARS-CoV-2 immunity. A noticeable difference between groups was observed after the second mRNA dose. Individuals with no prior exposure showed a more significant increase in their immune response after the second dose, but this was not observed in recovered individuals.
Specifically, IgG antibodies specific to the receptor-binding domain were similar between groups 1 week after the second dose. For recovered individuals, the first dose produced more neutralizing antibodies. However, the second dose did not add increased protection in the form of neutralization titers against D614G or the B.1.351 variant. After the first shot, naïve individuals showed a significant rise in memory B cells targeting the spike protein or receptor-binding domain.
Memory B cell levels further boosted after the second dose with levels comparable to non-vaccinated recovered individuals. People recovering from prior infection boosted memory B cells after the first shot. However, there was no change after receiving the second dose.