What does it imply that somebody has caught COVID-19 for a second time?

The case in Hong Kong features a 33-year-old man who was first diagnosed with COVID-19 in March, recovered, then tested positive again after traveling to Europe in July. There are multiple reasons to believe that this genuinely represents a case of reinfection. For one thing, the four and a half month interval between cases is long even for this lingering virus. Even more definitively, tests showed that the patient had been infected by two different strains of the virus with a number of genetic differences.

So … what does this tell us? Nothing that was really unexpected. There have been studies for several months now indicating that people who had mild or asymptomatic cases of COVID-19 developed low levels of antibodies against the virus. Those studies also show that the level of antibodies drops significantly over a period of two to three months. There’s far more to immunity than antibodies. It’s far more important that the body be prepared to manufacture antibodies, and mount other defenses, in case of encountering the virus again rather than walking around with a full load of antibodies ready to go. We also still don’t know how much antibody presence is needed to represent a good level of immunity.

Still, we know that mild cases produce low levels of antibodies, and that those low levels can drop after a few weeks. That seems to be related to what happened with the single patient from Hong Kong. The man at the center of the study had only a mild case of COVID-19 the first time around, so it’s likely his body never developed a high level of antibody response. Four and a half months later, he encountered the virus a second time and was reinfected.

That’s clearly not good—in the sense that it would be great if everyone, even asymptomatic carriers—developed an immune response that would provide a lifetime of protection. But there are reasons why it’s far from terrible.

First off, this is one man. He’s clearly not the only one; he just happened to be a case that was documentable because of the genetic fingerprint of his two infections, But this kind of thing does not appear to be happening at very high levels. Reinfection by the SARS-CoV-2 virus appears to be relatively rare.

Second, most of the vaccines now being trialed produce a level of antibody response that is well above that developed by patients with mild cases of COVID-19. If the reinfection occurred because the man in the study had a weak immune response to a mild case, then vaccines appear set to deliver a stronger reaction that should be more effective over a longer period. Vaccines against coronavirus may become an annual event rather than being one-and-done for life, but people shouldn’t need to get a booster every few months. That’s especially true when, hopefully, widespread use of vaccine drives the number of cases down to the point where tracing and case management can be effective.

Third, there is some genuine relief in the study. While the man had a mild case of COVID-19 initially, the second infection was even milder—in fact, asymptomatic. This suggests that, even though the first case was mild, the man still retained some immune response to the virus. It also suggests that SARS-CoV-2, like most viruses, hits with less impact when someone is infected again. This isn’t true of all diseases; dengue fever is famously relatively mild the first time someone catches it, and only earns its nickname of “break bone fever” on subsequent infections. That doesn’t appear to be the case with COVID-19, thank goodness.

At this point, reinfection with COVID-19 appears to be uncommon. It may be limited to patients who had a mild or asymptomatic case initially. It may be milder on reinfection than on first infection.

Though, honestly, drawing any conclusions from a single case is extremely presumptive. Unless and until additional such cases are identified (and assuming this paper survives peer review), it’s hard to draw any conclusions. 

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