LAUSR

It’s one of the last things you want to see when you’re pregnant. My husband was shivering next to me in bed, his forehead burning. “I think you have the flu,” I told him. “I can’t,” he said. “I got a shot.” But he did have the flu. And pretty soon, so did I. It’s not uncommon for flu shots to be less effective than intended. The 2014−2015 flu season, during which I got sick, was severe, according to the U.S. Centers for Disease Control and Prevention (CDC). The CDC estimated that the flu sickened 40 million people and sent 970,000 to the hospital in that period. The flu vaccine was only 23% effective against the common influenza type A and B viruses that season. For the H3N2 virus, a subtype of influenza A, which was the most prevalent virus that circulated, the vaccine was just 13% effective. The CDC attributed the low effectiveness to a phenomenon called drift. More than 80% of the circulating H3N2 viruses the CDC saw in patient samples in the 2014− 2015 season were different, or had “drifted,” from the viruses at which manufacturers had aimed their vaccines. The tendency of a flu virus to evolve has to do with how it replicates. Influenza uses its own RNA polymerase enzymes to replicate, according to Matthew Miller, a virologist and immunologist at McMaster University. “This makes the virus particularly prone to errors,” he says, meaning that the new viruses produced during replication can look different to our immune systems than the original viruses. Recent evidence has emerged that the efficacy of flu shots can also be affected by how the pharmaceutical industry makes its vaccines. Most flu shots are currently produced from viruses grown in chicken eggs containing an embryo. Scientists have observed that mutations can occur in influenza viruses grown inside such avian cells, potentially leading to vaccines that are less effective because they are aimed at a slightly different version of the virus than the one we’re exposed to. Not everyone believes that such mutations cause enough of a drop in vaccine effectiveness to justify abandoning egg-based vaccines, however, and they are calling for more studies.