Vaccines can in some cases trigger more serious illness when exposed to an unrelated virus, via a process known as virus interference. A question that has lingered since the 2009 mass vaccination campaign against pandemic H1N1 swine flu is whether seasonal influenza vaccination might make pandemic infections worse or more prevalent. That concern has come into sharp focus as studies raise questions about vaccine rollouts for COVID-19 and research points to the potential of problematic viral interference. To help understand that, it’s helpful to first understand a more common, beneficial virus interference. This kind can be a boost to our fight against invading pathogens. Normal viral interference occurs when a prior viral infection offers neighboring cells a kind of protection. In a way, the original invader blocks the subsequent viral invaders from taking hold. A study published in March in the Journal of Infectious Diseases reveals one example. Researchers with the …