A new preprint study released by B.C. researchers suggests that a single dose of an mRNA COVID-19 vaccine reduced the risk of infection in seniors by two-thirds during B.C.’s third wave in the spring.
The study from B.C. Centre for Disease Control researchers, which has not yet been peer-reviewed, looked at the effectiveness of B.C.’s vaccination plan, which involved delaying the second dose in order to get a first shot out to more people. The gap between first and second doses in B.C. began at 16 weeks but has recently shrunk down to eight.
The study looked at 16,993 people, of whom seven per cent tested positive for COVID and 93 per cent tested negative. Samples were collected from seniors age 70 and up who lived in the community and received their first dose of a mRNA vaccine at least 21 days earlier. The collection period started on April 4 and ended on May 1.
According to the study, vaccine effectiveness was “negligible” at 14 per cent between zero and 13 days after the first dose. Between 14 and 21 days post-shot, effectiveness rose to 43 per cent and by 35 to 41 days, it increased sharply to 75 per cent. On average, effectiveness was at 65 per cent overall at any period between 21 and 45 days.
The study found that effectiveness did shrink for variants of concern; 21 or more days post shot, vaccines had an effectiveness of 72 per cent against non-variant COVID, 67 per cent for B.1.1.7 (alpha, U.K. origin) and 61 per cent for P1 (gamma, Brazil origin).
About 45 per cent of the COVID-positive samples collected were of B.1.1.7 (alpha, U.K. origin), 28 per cent were P1 (gamma, Brazil origin) and 24 per cent were non-variant of concern. The study did not look at the delta (B.1.617) variant.