madeleineshort

When do booster effects kick in?

Question: How long after a booster shot until a person is most protected? I assume there is some protection immediately and full effect after 2 weeks. 

Answer: Booster effectiveness increases over time. It is meaningfully improved 7+ days after booster receipt, and further improved 14+ days after receipt. Here’s a snapshot of what we currently know:

  1. Results from a large observational study from Israel of individuals ages 60+ were published in New England Journal of Medicine last week. Protection of BNT162b2 Vaccine Booster against Covid-19 in Israel shows that effectiveness is stronger at 12+ days post-3rd dose as compared to 4-6 days after. “At least 12 days after the booster dose, the rate of confirmed infection was lower in the booster group than in the nonbooster group by a factor of 11.3 (95% confidence interval [CI], 10.4 to 12.3); the rate of severe illness was lower by a factor of 19.5 (95% CI, 12.9 to 29.5). In a secondary analysis, the rate of confirmed infection at least 12 days after vaccination was lower than the rate after 4 to 6 days by a factor of 5.4 (95% CI, 4.8 to 6.1).”  [for a refresh on 95% confidence intervals, see Q&A of 3/16/21]   
  2. A large observational study from Israel, Short Term Reduction in the Odds of Testing Positive for SARS-CoV-2; a Comparison Between Two Doses and Three doses of the BNT162b2 Vaccine (pre-print) studied COVID infection rates among members of Israel’s second largest health management organization ages 40+ who received either two or three doses of Pfizer.  Researchers found that the booster shot’s marginal effectiveness increased over time “with no marginal effectiveness in the first 7 days (3%; 95% CI: -5%, 10%), moderate marginal effectiveness in days 7-13 (48%; 95% CI: 42%, 54%), and high marginal effectiveness in days 14-20 (79%; 95% CI: 72%, 84%).” (Figure 1)
  3. A small clinical trial conducted by Pfizer that examined immune response over time found that neutralizing titers were much higher 7 days and even higher 1 month post- 3rd dose as compared with that period post-2nd dose. 

Figure 1. Marginal Effectiveness of 3rd dose booster over time (from Patalon et. al)