Whether you get it from a vaccine or an infection, COVID-19 immunity does not last forever. In a study published May 3 in JAMA Network Open, researchers combed through studies to determine just how long protection from the shots endures.

The scientists, led by a team from Italy, analyzed 40 studies that documented people’s vaccination status and their subsequent infections with COVID-19, confirmed by lab tests. The studies included data from both the Delta and Omicron surges.

Overall, the researchers found that one month after people received two doses of either mRNA vaccine (from Moderna or Pfizer-BioNTech), the vaccine from AstraZeneca, or the shot from Sinovac, the vaccine effectiveness was 53% in protecting against symptoms of COVID-19. (There were differences among the vaccines, with Moderna’s primary series of two shots showing the highest effectiveness of 62% one month after the series, and Sinovac’s demonstrating the lowest effectiveness at 32%.) After six months, the overall effectiveness of the vaccines dropped further to 14%, and to 9% after nine months. This waning was greater during the Omicron wave than during the Delta wave, suggesting that the vaccine was less effective against Omicron.

Booster doses after the primary series restored protection back to levels achieved just after the primary vaccination, but this protection waned too, at a rate similar to that after the primary series, dropping from 60% at one month after the booster dose to 13% at nine months. This drop in efficacy corresponded to rates of lab-confirmed positive tests for Delta and Omicron infections.

Among 146,243 tested contacts of 108,498 index patients, 54,667 (37%) had positive SARS-CoV-2 polymerase-chain-reaction (PCR) tests. In index patients who became infected with the alpha variant, two vaccinations with either BNT162b2 or ChAdOx1 nCoV-19 (also known as AZD1222), as compared with no vaccination, were independently associated with reduced PCR positivity in contacts (adjusted rate ratio with BNT162b2, 0.32; 95% confidence interval [CI], 0.21 to 0.48; and with ChAdOx1 nCoV-19, 0.48; 95% CI, 0.30 to 0.78). Vaccine-associated reductions in transmission of the delta variant were smaller than those with the alpha variant, and reductions in transmission of the delta variant after two BNT162b2 vaccinations were greater (adjusted rate ratio for the comparison with no vaccination, 0.50; 95% CI, 0.39 to 0.65) than after two ChAdOx1 nCoV-19 vaccinations (adjusted rate ratio, 0.76; 95% CI, 0.70 to 0.82). Variation in cycle-threshold (Ct) values (indicative of viral load) in index patients explained 7 to 23% of vaccine-associated reductions in transmission of the two variants. The reductions in transmission of the delta variant declined over time after the second vaccination, reaching levels that were similar to those in unvaccinated persons by 12 weeks in index patients who had received ChAdOx1 nCoV-19 and attenuating substantially in those who had received BNT162b2. Protection in contacts also declined in the 3-month period after the second vaccination.