As many of us are nearing the one-year mark following our immunization, questions still remain regarding the long-term efficacy of our current vaccines. New research, however, is looking into it.

A team of researchers from the Beth Israel Deaconess Medical Center (BIDMC) has been analyzing the long-term immunization efficacy of the three vaccines approved by the U.S. Food & Drug Administration in December 2020. These are BNT162b2 (BioNTech, Pfizer), mRNA-1273 (Moderna), Ad26.COV2.S (Johnson & Johnson).

They evaluated the immune response produced by these vaccines at two to four weeks after complete immunization (i.e. after receiving the full number of shots) to that at eight months after vaccination.

Declining but not determined

“The mRNA vaccines were characterized by high peak antibody responses that declined sharply by month six and declined further by month eight,” said corresponding author Dan H. Barouch, MD, Ph.D., director of the Center for Virology and Vaccine Research at BIDMC, who helped develop the Ad26 platform in collaboration with Johnson & Johnson.

“The single-shot Ad26 vaccine induced lower initial antibody responses, but these responses were generally stable over time with minimal to no evidence of decline.”

Understanding the long-term efficacy of these vaccines is critical for our efforts to combat the COVID-19 pandemic. However, we didn’t have such information on hand up to now. Simply put, while the vaccines were tested to ensure safety and efficacy, the global context meant that their development process was greatly accelerated. We simply didn’t have the opportunity to obtain data pertaining to their long-term efficacy.

In a bid to help patch up this hole in our understanding, the team at BIDMC monitored the immunization levels of 61 participants over an eight-month period after they received their vaccines. The team measured the levels of antibodies, T cells, and other immune markers in the blood of these participants at two to four weeks after they received their shot (which is the point of peak immunity) and monitored them over an eight-month follow-up period.

Out of the 61 total participants involved, 31 received the BioNTech / Pfizer vaccine, 22 received the Moderna one, with the final 8 receiving the Johnson & Johnson single-shot vaccine.

All in all, the team explains that the Moderna vaccine produced more powerful and longer-lasting immunization effects than the BioNTech / Pfizer variant. That being said, all three variants produced effective immune responses against SARS-CoV-2 and had broad cross-reactivity to its strains.

However: the authors report that both mRNA-based vaccines (BioNTech / Pfizer and Moderna) produced sizable initial immune responses, but these got progressively weaker over time. At around the 6-month mark, immune markers in patients who received either of these two had already declined sharply compared to the 2-to-4 week mark. The same markers would decline even further at the eight-month mark.

The single-shot Johnson & Johnson vaccine, meanwhile, produced a weaker initial effect but was much more consistent over the study period.

Although these results might not sound very exciting or promising, they do not mean that the vaccines leave us vulnerable over time. For starters, there are still a lot of unknowns regarding exactly what immune responses in our bodies are needed to protect against SARS-CoV-2.

Furthermore, what the team tracked here are physical markers of immunity. But the antibodies themselves, for example, are the ‘soldiers’ that our body uses to protect itself against viruses. Their presence in the bloodstream is akin to our body being on alert. But even if they are not physically there, our bodies have already been primed regarding the structure of the virus, how to identify it, and which antibodies are needed to defeat it. Against this backdrop, an immune response against the pathogen can be mounted very quickly in case of infection.

“Even though neutralizing antibody levels decline, stable T cell responses and non-neutralizing antibody functions at 8 months may explain how the vaccines continue to provide robust protection against severe COVID-19,” said lead author Ai-ris Y. Collier, MD, a maternal-fetal medicine specialist at BIDMC.

“Getting vaccinated (even during pregnancy) is still the best tool we have to end the COVID-19 pandemic.”

The paper “Differential Kinetics of Immune Responses Elicited by Covid-19 Vaccines” has been published in the New England Journal of Medicine.