(Reuters) – Increased contact among vaccinated people can give the false impression that COVID-19 vaccines are not working, researchers warn.
Some studies have suggested that vaccinated individuals are becoming infected at higher rates than unvaccinated individuals, but these studies are likely to involve statistical errors, particularly if they did not account for different contact patterns among vaccinated vs unvaccinated people, said Korryn Bodner of St. Michael’s Hospital, Unity Health Toronto.
Using computer models to simulate epidemics with a vaccine that protects against infection and transmission, her team identified conditions that could create “a perfect storm for observing negative vaccine effectiveness even when a vaccine was efficacious,” Bodner told Reuters.
For example, effective vaccines could appear ineffective when vaccinated people have more contact with each other than with unvaccinated people, when vaccine benefits become lower but are not absent (as has happened with new SARS-CoV-2 variants), or when effectiveness is measured while an epidemic is growing (such as when a new variant is emerging), according to a report posted on medRxiv ahead of peer review.
The simulations do not prove that this type of bias affected studies of vaccine effectiveness versus the Omicron variant. They show, however, that “even if vaccines work, increased contact among vaccinated persons can lead to the appearance of the vaccine not working,” Bodner said.