The first line of vaccines was highly effective at restricting COVID-19’s damage

• COVID-19 vaccination was deemed crucial to prevent the continued spread of the disease, protect those infected from experiencing severe effects, counter the rise of new variants, and ultimately end the pandemic.
• Understanding the effectiveness of vaccines remains crucial.

Primary doses and boosters

• A total of 5.09 billion people have completed a primary COVID-19 vaccination series (i.e., two doses of a two-dose vaccine or one dose of a one-dose vaccine).
• First, how well does the primary series of COVID-19 vaccines protect people (against infections, hospitalizations and deaths) four months or more after completing vaccination?
• Answering these questions will provide invaluable information for policymakers to make evidence-based decisions, such as the timing of administering COVID-19 vaccine booster doses.


In total, we identified 68 studies that met these criteria, representing 23 countries. We then combined all the data to better understand how the vaccines’ protection changes over time. The results were published in Lancet Respiratory Medicine.

Protection against COVID-19, in general

• Specifically, vaccines should show at least 70 per cent protection against infections and 90 per cent protection against hospitalizations and deaths.
• We found that the primary series offered excellent protection against hospitalizations and deaths in the short term, showing over 90 per cent protection against both outcomes within 42 days after vaccination.
• Protection then fell to around 45 per cent against infections and to around 70 per cent against hospitalizations after four months had passed.

Protection against the Omicron variant

• Vaccines were generally less effective against the Omicron variant, which emerged in fall 2021, about a year after COVID-19 vaccines were introduced.
• The primary series’ protection against hospitalization for Omicron infections reached around 70 per cent within the first 42 days, but also dropped over time, reaching closer to 50 per cent after six months.
• Yet, booster protection also waned over time, falling to about 40 per cent against Omicron infections and 70 per cent against hospitalizations after four months post-booster.
• With Omicron, boosters are particularly needed to maintain adequate protection, but this protection also needs additional boosting as it wanes over time.

Behaviour-based prevention measures remain necessary

• That means measures like wearing a mask, washing one’s hands, and staying at home when sick remain essential complements to vaccination.
• Contrary to vaccines, these measures do not decline in effectiveness over time and are particularly well suited to protect people against infections.
• Keven Joyal-Desmarais receives funding from the Fonds de recherche du Québec (FRQ) and the Canadian Institutes of Health Research (CIHR).