A team of McMaster University researchers led by Darryl P. Leong and MyLinh Duong has found that extending the dosing interval for a COVID-19 Pfizer vaccine can lead to a stronger immune response against the virus.

The study, which compared three dosing intervals for the primary vaccination among healthcare workers, found that extending the interval beyond 42 days significantly increased the presence of antibodies against the virus that causes COVID-19.

“This study data can help inform optimal dosing intervals to better fight COVID-19, “said Leong, an associate professor in the Department of Medicine. 

“This also provides reassurance about the decision to increase the vaccine dosing intervals compared with the original clinical trials.”

The researchers discovered that the Immunoglobulin G (IgG) antibody, which is found in the blood and bodily fluids, had a stronger response against the COVID-19 virus and was effective for longer periods of time. The IgG response was also found to be greater than comparable antibodies known as Immunoglobulin A (IgA) and Immunoglobulin M (IgM).

Data from Britain and Canada also says that delaying the second dose to 12 weeks is also associated with significantly lower COVID-19 transmission and symptomatic infections.

The McMaster study included a large sample size of healthcare workers who were followed for up to nine months after the second dose. It showed that a longer dosing interval can provide higher antibody levels beyond three months, offering better immune protection against newer variants like Omicron.

However, the research team caution there may be a higher risk of infection following the first dose with longer dosing intervals, particularly with highly transmissible variants, as immunity further wanes with longer intervals between doses.

The study was published in the journal PLoS One.