The researchers found the vaccines developed prior to Omicronâs emergence provided an 83 per cent effectiveness against infection at baseline, i.e., between 14 to 42 days. Effectiveness decreased to 62 per cent by 112 to 139 days (16 to 20 weeks). That number dipped to only 47 per cent by 280 days (40 weeks).
The vaccines were 92 per cent effective against hospitalizations at baseline. That figure decreased to 79 per cent at 224 to 251 days, or 32 to 36 weeks.
Regarding mortality, the vaccines were 91 per cent effective in preventing deaths at baseline. Their rate of effectiveness changed to 86 per cent at 168 to 195 days (24 to 28 weeks).
Booster doses showed reasonably stable long-term protection against hospitalizations and mortality but also similar rates of declining effectiveness. Booster protection against infection was more modest, performing slightly below WHO guidelines at baseline.
The researchers also note that messenger RNA (mRNA) vaccines developed by Moderna and Pfizer-BioNTech were more effective against infections than the adenovirus vaccines from AstraZeneca/Oxford and Janssen. Still, both types of vaccine provided similar levels of protection against hospitalizations and mortality.
The Omicron variantâs emergence in 2021 upended these numbers, says Bacon. Baseline protection against infection dropped to around 67 per cent and diminished rapidly. However, he notes that the primary vaccines were better at protecting people against hospitalizations and mortality.
âIf we really want to get on top of the pandemic and stop infections when they start spiking, we probably need something more than just the vaccine,â Bacon points out. He says masking and social distancing remain the best ways to reduce infections and halt the spread of the virus.
Bacon adds that he realizes local governments and society at large have largely moved on from those early pandemic policies, which are unlikely to return without a sharp increase in infections. He warns that now is not the time to stop thinking about COVID-19 or to disregard public health measures altogether.
âIf you are very well-vaccinated and you are low-risk, then you probably will not end up in hospital or dying from COVID-19,â he comments. âHowever, people should be conscious of people of higher risk, and of interacting with each other. If you are a person who is low risk and become infected with the virus, you will probably be fine. But a vaccine alone will not prevent you from possibly transmitting the virus to someone who is high risk. And that person may end up with severe consequences.â
This study received financial support from the Canadian Institutes of Health Research and the Public Health Agency of Canada.
Read the cited paper: âLong-term effectiveness of COVID-19 vaccines against infections, hospitalisations, and mortality in adults: findings from a rapid living systematic evidence synthesis and meta-analysis up to December, 2022â