Two preliminary studies show that for individuals with preexisting immunity to SARS-CoV-2, the first dose of vaccine is likely to be the immunologic equivalent of the booster dose in naïve individuals. In addition, individuals with preexisting immunity also experience more severe reactogenicity after the first few doses than naïve individuals. This raises the question of whether individuals with preexisting immunity should also take the second dose of vaccine.
Phase-3 studies of the anti-COVID BNT162b2/Pfizer and mRNA-1273/Moderna vaccines reported high efficacy in preventing symptomatic SARS-CoV-2 infections after two doses of vaccine administered 3 to 4 weeks apart (BNT162b2: 21 days; mRNA-1273: 28 days) in individuals without antibodies to the virus. Two preprint articles, thus not yet peer-reviewed, describe the outcomes of administering a single dose of anti-COVID vaccine in subjects previously infected with SARS-CoV-2 virus. Both studies suggest that a single dose of mRNA vaccine elicits very rapid immune responses in HIV-infected individuals, with post-vaccine antibody titers comparable to or even higher than those found in naïve individuals who took two doses of vaccine.
Professor Eleanor Riley, professor of immunology and infectious diseases at the University of Edinburgh (UK), said, "The data presented in these two articles are not surprising, but they are very reassuring. Both articles show that a previous SARS-CoV-2 infection prepares the immune system to give a very robust response to a single dose of COVID-19 vaccine. 14 days after a single dose of vaccine, subjects with previous COVID-19 infection confirmed by RT-PCR testing, or who showed preexisting anti-COVID antibodies consistent with having had a previous infection, had antibody concentrations that were as high as (or up to 10 times higher than) the levels seen in previously uninfected persons who had taken two doses of vaccine (Krammer et al) and 10 times higher than persons who had been hospitalised for severe COVID-19 (Saadat et al). These data indicate that vaccines very effectively increase infection-induced immunity.
The authors of both articles suggest that people who have had SARS-CoV-2 infection confirmed by RT-PCR testing can take only one dose of vaccine. It appears that this may provide them with protection that is at least as good as two doses of vaccine. However, incorporating this into a mass vaccination program may be logistically complex and it may be safer, in general, to ensure that everyone receives two doses.
The article by Krammer et al. also showed that people who were infected before vaccination were more likely to have flu-like symptoms (fatigue, chills, headache) in the days following their vaccination, suggesting that their cellular immune system was also re-stimulated.
Prof. Lawrence Young, virologist and professor of molecular oncology, University of Warwick (UK), said, "Is a single dose of vaccine sufficient in individuals previously infected with the SARS-CoV-2 virus? This preprint shows that the antibody response to the virus after the first dose of an mRNA vaccine in individuals with pre-existing immunity due to previous infection is equal to or greater than that induced by two doses of the vaccine in uninfected individuals. It also shows that side effects are significantly higher in response to vaccination if there has been a previous infection. Antibody levels in previously infected individuals elevated rapidly in response to a single vaccination reaching 10-20 times those observed in those who were vaccinated without evidence of prior infection.”
Prof. Young added that “those who took the vaccine with preexisting immunity experienced systemic side effects with significantly greater frequency than individuals without antibodies (eg, fatigue, headache, chills, fever, muscle or joint pain). This suggests that the first dose of vaccine serves as a boost in previously infected individuals, giving levels of protective immunity equivalent to those achieved with two doses of vaccine in individuals who have not been previously infected. This means that we should do additional studies to evaluate the efficacy of a single dose of vaccine in previously infected individuals. If future studies confirm this high level of immunity after a single mRNA vaccine in this group of individuals, we would have a viable option when it comes to problems with the vaccine supply."