Pfizer and Moderna COVID-19 Vaccines Found to Induce Robust T Cell Responses Against Variants

As vaccinations continue to ramp up worldwide in the fight against COVID-19 amidst uncertainties over variants of concern, early data from a lab study shows that the mRNA COVID-19 vaccines from Pfizer and Moderna are effective against some of the variants.

In a small study conducted by researchers from the University of California and the Gladstone Institute of Virology in San Francisco, it was found that both vaccines elicited immune responses, particularly robust T cell responses, against the UK B.1.117 and South African B.1.351 variants.

The study has been published in the online pre-print journal bioRχiv.

The researchers focused on the T cell response as it is key to long-lasting immunity. They found that the vaccines stimulated generation of T cells specific to the spike protein of SARS-CoV-2 (the virus that causes COVID-19) against both B.1.117 and B.1351 in similar numbers and phenotypes.

The investigators say the results provide reassurance that vaccine-elicited T cells respond robustly to both the B.1.1.7 and B.1.351 variants.

Related: Pfizer COVID-19 Vaccine Trial Results: Stellar Efficacies in Children and Against Variants

The study revealed that the second dose of either vaccine boosted the quantity but not quality of the T cell response.

The researchers also examined vaccine-induced immunogenicity in individuals previously infected with COVID-19 (referred to as convalescents). They found that while one dose is beneficial in convalescents, a second dose does not improve T cell numbers, and thus offers no added value.

The researchers conducted a longitudinal study of infection-naïve and COVID-19 convalescent donors before vaccination, and after their first and second vaccine doses. They used multi-parameter CyTOF analysis to phenotype SARS-CoV-2-specific T cells induced by vaccination.

The study involved evaluating blood samples for the T cells from eight individuals collected at three time points (for a total of 24 samples): prior to vaccination, about two weeks after the first dose of vaccine, and about 2 weeks after the second dose. Three of the participants had received the Moderna vaccine and the other five had received Pfizer. Half of the cohort had never been infected with SARS-CoV-2 while the other half had undergone complete recovery from mild COVID-19 disease (no hospitalization).

The researchers first identified SARS-CoV-2-specific T cell populations in the 24 samples. They then measured T cell responses to the spike proteins of the original strain of SARS-CoV-2 and B.1.117 and B.1.351 by exposing them to 15-mer peptides of the full-length spike protein from each strain in the samples.

Interestingly, it was found that spike-specific T cells from previously infected, convalescent vaccine recipients were strikingly different from those of infection-naïve vaccinated individuals, with phenotypic features suggesting superior long-term persistence and ability to home to the respiratory tract including the nasopharynx, say the researchers. This suggests that vaccinated convalescents may have SARS-CoV-2-specific T cells that persist and specifically reside in the nasopharynx compared to infection-naïve individuals.

This report backs up a study conducted by Pfizer, BioNTech and the University of Texas which showed that the Pfizer/BioNTech vaccine was effective against three variants including the P.1 Brazilian strain.

In addition, Moderna released results of a Phase II study earlier this month that showed its new, slightly modified booster shot designed against B.1.351 provided additional protection against the original strain of the virus as well as the P.1. and B.1.351 variants.