Vancouver, BC—Since emergency use authorization was granted by the FDA for the messenger RNA (mRNA) COVID-19 vaccines, dosing intervals have not changed much: About 3 weeks between Pfizer-BioNTech shots and 4 weeks for Moderna.

Now, a new peer-reviewed study based in North America has examined the timing between the first and second doses of COVID-19 mRNA vaccines. The report in Clinical Infectious Diseases Indicates that a longer dose interval leads to a stronger immune response.

The study's principal investigator, Brian Grunau, MD, MHSc, of the University of British Columbia, suggests that the results provide valuable information for ongoing international COVID-19 vaccination efforts.

The study, funded by the Canadian government, compared blood test results from 186 paramedics, some of whom were vaccinated within the earlier recommended interval of fewer than 4 weeks, and others who received their second doses after 6 to 7 weeks.

"We found significantly higher levels of antibodies in individuals who had longer vaccine intervals, and this was consistent regardless of which mRNA vaccine was administered," Dr. Grunau advises.

"The optimal dosing interval for severe acute respiratory syndrome coronavirus 2 vaccines remains controversial," the authors write. "In this prospective study, we compared serology results of paramedics vaccinated with mRNA vaccines at the recommended short (17–28 days) vs long (42–49 days) interval. We found that a long dosing interval resulted in higher spike, receptor binding domain, and spike  N terminal domain antibody concentrations."

While SARS-CoV-2 vaccines have been determined to be highly efficacious in preventing symptomatic disease, that is based on vaccine administration schedule that usually involves 3 or 4 weeks between vaccine doses, according to the authors, who add, "This may not be the optimal schedule at the individual level to achieve robust and long-lasting immunity or at the population level to achieve the fastest and overall community-level protection."

To find out, the study team investigated the differences in immune response according to vaccine dosing intervals of fewer than 4 weeks versus 6 to 7 weeks among paramedics who received mRNA vaccines.

Samples were included from COVID-19 Occupational Risks, Seroprevalence and Immunity among Paramedics in Canada study participants. All received two doses of either the BNT162b2 (Pfizer), 70.4%, and/or mRNA-1273 (Moderna) vaccines, 29.6%. Median age of the paramedics was 38 years (interquartile range [IQR], 33-45). Blood sampling followed the second vaccine dose by a median of 56 days (IQR, 29-76).

"This longer interval strategy enables early access to first doses in the population and ensuring the best protection possible with the two-dose series," Dr. Grunau added.

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