Grunau B, Goldfarb DM, Asamoah-Boaheng M, et al. - The immunogenicity of extended mRNA vaccine dosing intervals was examined.

• Canadian paramedics were recruited in the COVID-19 Occupational Risks, Seroprevalence and Immunity among Paramedics in Canada cohort study (approved by the University of British Columbia and University of Toronto research ethics boards).

• This analysis was performed including individuals who provided a blood sample at enrollment or between 170 to 190 days after the first dose and had received 2 mRNA vaccine doses.

• The observed variability of vaccine intervals and timing of enrollment were relied upon when compared with vaccination to select participants with different vaccine intervals and two separate investigations were conducted, with different approaches to timing of blood samples.

• In the first investigation, antibody levels were compared at comparable time intervals after the second dose.

• For comparing the short (≤ 28 days) vs medium (42-49 days) vaccine dosing interval, researchers selected 30 samples (collected at enrollment) from each group based on similar second vaccine-to-sample-collection intervals and matched them by vaccine type, age, gender, and comorbidities (eAppendix in the Supplement).

• In the second investigation, researchers compared antibody levels sampled at a standardized interval (170-190 days) after the first dose.

• For comparing the short (≤ 36 days) vs long (100-120 days), selection of 30 samples from each group was done, matching by the same characteristics.

• Findings revealed improved immunogenicity with longer mRNA vaccine dosing intervals; this was consistent when responses were determined on the basis of the timing of the first or second dose.

• In particular, extending dosing intervals appeared advantageous against the Delta variant.

• With the delayed second-dose strategy, a larger proportion of the population may have induction of faster partial protection when vaccine supplies are limited.