COVID-19 Cellular Immunity Persists at Least 6 Months after mRNA Vaccination

Previous research suggested vaccine efficacy significantly dips after 6 months, but a new study finds COVID-19 cellular immunity remains strong.

In a statement released today, Johns Hopkins Medicine shared study data showing CD4+ T lymphocytes in people who received an mRNA COVID-19 vaccine persist for 6 months.

The levels of CD4+ T lymphocytes, also called helper T cells, were only slightly lower after 6 months than they were 2 weeks after vaccination, and are significantly higher than in unvaccinated persons.

Joel Blankson, MD, PhD, professor of medicine at the Johns Hopkins University School of Medicine and the study’s senior author further expounded these findings: “Previous research has suggested that humoral immune response — where the immune system circulates virus-neutralizing antibodies — can drop off at six months after vaccination, whereas our study indicates that cellular immunity — where the immune system directly attacks infected cells — remains strong.”

Blankson and his team obtained blood samples from 15 participants on 3 occasions: before vaccination, 7-14 days after their second dose of the mRNA Moderna or Pfizer-BioNTech COVID-19 vaccine, and 6 months after vaccination. The average participant age was 41, 10 participants were men and 5 were women, and none had a prior history of COVID-19 infection.

The CD4+ T lymphocytes the investigators studied also protect against the COVID-19 Delta variant. “The persistence of these vaccine-elicited T cells, along with the fact that they’re active against the delta variant, has important implications for guiding COVID vaccine development and determining the need for COVID boosters in the future,” Blankson said.

The investigators found the number of helper T cells recognizing COVID-19 Spike proteins to be extremely low prior to vaccination, at an average of 2.7 spot-forming units (SFUs) per million peripheral blood mononuclear cells (PBMCs). In the 7-14 days after an mRNA vaccine series, T cell frequency rose to 237 SFUs per million PBMCs. At the 6-month mark, the levels dipped to 122 SFUs per million PBCMs.

The helper T cell frequency decreased after 6 months, but remained significantly higher than before COVID-19 immunization. The number of T cells recognizing the Delta variant’s Spike protein was not significantly different than the T cells attuned to the protein of the original virus strain.

“The robust expansion of T cells in response to stimulation with spike proteins is certainly indicated, supporting the need for more study to show booster shots do successfully increase the frequency of SARS-CoV-2-specific T cells circulating in the blood,” Blankson said.