You’ve probably heard antibody levels can drop quickly, even to undetectable, within a few months of recovery from Covid. Does that mean people aren’t immune? Not necessarily- here’s a quickly analogy involving police chasing down bad guys to explain why. If our immune cells... 
...are cops chasing SARS2 throughout our bodies, then think of antibody levels as being their speed. The more antibodies, the faster our cops are going, and the easier they can chase down the bad guy virus clones in their turbo charged Lamborghinis.
When a person is first infected, the virus hits the highways and starts picking up speed (replicating in our cells) but the cops aren’t even in their cars. They don’t know what the bad guy looks like, what car he’s driving, and their car engines are cold.
Once the cops realize that there’s an infection and figure out what it is, they jump in their cars, turn the ignition, warm their engines, and hit the gas. As they speed up (generate antibodies), they start to overtake the bad guys and stop them.
Eventually, they eradicate the virus. Some of them continue to patrol the streets (some people still have antibodies) but in some people the cops basically seem to pull over. In this analogy, all cops being at a dead stop means there are no detectable antibodies.
If looking for police cars driving on road, you won’t find any. However, that does not mean that person is unprotected. Cops are now sitting in their cars, engines on & warmed, w/ their foot on the gas, keeping eye out for bad guy knowing exactly what he looks like: SARS2.
So as soon as a SARS virus shows up driving a Lamborghini down the streets of the city, multiple cops hit the gas and catch up quickly, likely stopping that virus before it can do nearly as much damage as the first time, when the cops were unprepared.
And that’s why, just because someone doesn’t have detectable antibodies after they recovered from covid, it doesn’t mean that they aren’t better protected against serious illness if they are exposed again. They might not even know they got exposed & might not spread it.
I’m not saying that people who recover and have or don’t have antibodies necessarily are protected from reinfection. I’m just saying that no antibodies doesn’t mean they aren’t. We’ll soon know much more.
All that applies to vaccines, which show cops what bad guy looks like. Adjuvants (immune stimulants) in vaccines make those cops patrol the streets. But even if they slow down or stop (antibodies drop/disappear), police remain in their cars at the ready & on lockout.
Booster shots then rev them up again so they are even more likely to stop the bad guy if one shows up racing down the street. Until vaccines come along, remember to social distance and mask up.
p.s. The times make such analogies fraught w/ complexity. Necessary biological functions can be both helpful & hurtful. Eg Auto-immune disorders are caused by some immune cells turning on tissues they are meant to protect. We must diagnose & treat to preserve proper function.
For frequently asked questions and lots of fun science explainers, be sure to see my pinned tweet with a ton of other threads.
