Early data indicated that the Pfizer BioNTech mRNA vaccine demonstrated 90 percent efficacy in preventing infection from the alpha variant of SARS-CoV-2, the virus that causes COVID-19. The vaccine is typically administered in two doses, delivered three to four weeks apart. However, in the setting of vaccine shortages in some areas of the United States, some public health agencies have opted to deliver the vaccines on an extended interval. Findings from a recent study suggest that the vaccine elicits robust humoral immunity even when doses are delivered 16 weeks apart.

Humoral immunity, also known as antibody-mediated immunity, is an aspect of the immune response to specific antigens. It occurs when foreign material, such as a pathogen, is detected in the body. Lymphocytes are the primary drivers of humoral immunity.

The authors of the study measured humoral responses to two doses of the Pfizer BioNTech vaccine, delivered 16 weeks apart. The study included 22 people who had never been infected with SARS-CoV-2 and 21 people who had been infected. Among those who had previously been infected, ten participants did not receive a second dose, leaving 11 recipients of two doses. The authors collected blood samples for testing prior to the first dose, three weeks after the first dose, three months after the first dose, and three weeks after the second dose.

They found that providing a second dose to previously infected people did not significantly improve humoral responses. However, humoral responses in people who had never been infected increased markedly after the delayed second dose, achieving levels comparable to those observed in previously infected people.

These findings suggest that delaying the second dose of the Pfizer BioNTech vaccine as much as 16 weeks provides robust humoral immunity against SARS-CoV-2 infection. Learn more about COVID-19 vaccines in this episode featuring Dr. Roger Seheult and Dr. Rhonda Patrick.

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