First, the virus. Fears of a bad flu season first began in the early fall, after public health officials noticed a worse-than-average flu season in the southern hemisphere. The dominant circulating strain this year is H3N2, which hits humans harder than other strains. Scientists don’t really know why, but flu seasons where H3N2 have dominated in the past have tended to be worse. STAT reporter Helen Branswell called it the “problem child of seasonal flu.”

Second, the vaccine. This year’s vaccine was only 10 percent effective against the problematic H3N2 strain in Australia.

Flu experts have the unenviable task of predicting circulating flu strains several months in advance—so that vaccine manufacturers have the time it takes to grow millions of doses in chicken eggs. This year, they got the dominant strain right. But even when flu experts do a decent job with their predictions, the other problem is the chicken eggs. Flu viruses can pick up mutations and evolve as they grow in bird cells, which are not their preferred environment. (After all, they want to infect humans.) So in the end, the flu viruses that end up in vaccines look a little different than the ones circulating out in the world. The H3N2 strain is especially prone to significant egg-induced mutations.