"It's not unusual to have fading antibody response after several months. The drop-off isn't that surprising. When you look at something like the smallpox vaccine, you see the antibody response is down about 75 percent after six months. But that's a vaccine that works for decades.
Read the whole article on The Atlantic here: https://www.theatlantic.com/ideas/archive/2020/07/could-covid-19-immunity-really-disappear-months/614377/.

