"The truth of the matter is that pandemics always end. And to date vaccines have never played a significant role in ending them. (That doesn’t mean vaccines aren’t playing a critical role this time. Far fewer people will die from Covid-19 because of them.) But there were no flu vaccines in 1918, when the world didn’t yet know that the great influenza was caused by a virus, H1N1. In 1957, when the H2N2 pandemic swept the world, flu vaccine was mainly a tool of the military. In the pandemic of 1968, which brought us H3N2, the United States produced nearly 22 million doses of vaccine, but by the time it was ready the worst of the pandemic had passed, and demand subsided. That “too little and too late” phenomenon played out again in 2009, when the world finally had the capacity to make hundreds of millions of doses of H1N1 vaccine; some countries canceled large portions of their orders because they ended up not needing them. How did those pandemics end? The viruses didn’t go away; a descendent of the Spanish flu virus, the modern H1N1, circulates to this day, as does H3N2. Humans didn’t develop herd immunity to them, either. That’s a phenomenon by which a pathogen stops spreading because so many people are protected against it, because they’ve already been infected or vaccinated. Instead, the viruses that caused these pandemics underwent a transition. Or more to the point, we did. Our immune systems learned enough about them to fend off the deadliest manifestations of infection, at least most of the time. Humans and viruses reached an immunological détente. Instead of causing tsunamis of devastating illness, over time the viruses came to trigger small surges of milder illness. Pandemic flu became seasonal flu."