Story at-a-glance
Vaccines can in some cases trigger more serious illness when exposed to an unrelated virus, via a process known as virus interference
 Virus interference was found to be at play during the 2009 pandemic swine flu. The seasonal flu vaccine increased people’s risk of getting sick with pandemic H1N1 swine flu and resulted in more serious bouts of illness
 Researchers have also found military servicemen vaccinated against influenza were more prone to unspecified coronavirus infection than unvaccinated counterparts
 An October 2020 data analysis found a positive association between COVID-19 deaths and influenza vaccination rates in elderly people worldwide. Areas with the highest vaccination rates also had the highest COVID-19 death rates
 Possible explanations include reduced immunity to SARS-CoV-2 by some unknown biological mechanism, and viral interference causing reduced nonspecific immunity