A single dose of mRNA vaccine may trigger variant-resilient immune responses

Emerging SARS-CoV-2 variants are a significant source of concern, especially the most resistant ones, such as the one originally identified in South Africa (B.1.351). In a publication in Science, CITF-funded researcher Dr. Andrés Finzi, from the Université de Montréal, helped a team from the Fred Hutchinson Cancer Research Center in Seattle find that after receiving a single dose of mRNA vaccine, participants who had previously been infected with SARS-CoV-2 were able to produce large amounts of antibodies capable of neutralizing all circulating variants, including B.1.351.

Recently, several viral variants of concern have emerged. The ones widely studied have been first identified in the United Kingdom (B.1.1.7), South Africa (B.1.351), and Brazil/Japan (P.1). They harbor specific mutations that may be associated with increased transmissibility. Several studies from around the world have described how B.1.351 is partially resistant to neutralizing antibodies made against previously circulating variants, suggesting current vaccines may not be as effective against this variant.

In this study, the Fred Hutchinson Cancer Research Center team, led by Dr. Andrew McGuire, suggest that the mRNA vaccines are effective against the B.1.351 variant after a single dose in people who had previously been infected with SARS-CoV-2. After following a long-running COVID-19 study in Seattle, Washington, they report that antibodies in pre-vaccination blood samples from people who recovered from SARS-CoV-2 infection, indeed neutralized the parent virus, the one initially isolated from China, Wuhan-Hu-1. However, antibodies in these pre-vaccination blood samples neutralized the more resistant newer variant B.1.351 to a lower extent.

Studies have shown that a previous SARS-CoV-2 infection generally results in much higher levels of antibodies after one dose of vaccine, even compared to people who were never infected but who received two vaccine doses. The authors of this publication report that after receiving a single dose of Pfizer-BioNTech or Moderna mRNA vaccine, participants who had previously been infected with SARS-CoV-2 were able to produce large amounts of antibodies capable of neutralizing B.1.351. Indeed, a single mRNA vaccine immunization boosted neutralizing titers against all variants tested and SARS-CoV-1, up to 1000 times. The authors also found, however, that a second dose of vaccine did not increase their neutralization capacity. This study showed that vaccines elicited an antibody response capable of neutralizing SARS-CoV-2 variants on those who had never had a previous infection, but at lower levels.

The authors conclude that most previously infected people will benefit from a single immunization with either the Pfizer-BioNTech or Moderna vaccine (it is unclear if other vaccines would elicit the same type of response) as it will lead to significant increases in neutralizing antibody responses against vaccine-matched and emerging variants. Furthermore, they suggest that a second dose administered three to four weeks later does not further boost neutralizing titers in previously infected individuals. They indicate that the second dose of an mRNA vaccine could be delayed for these individuals if monitoring of the neutralizing antibody titers before and after the first vaccine dose is possible.