Does this case actually prove that re-infection with the virus is possible? Or was this just a mistake in the testing? Or did the person have a particularly weak immune system so that she couldn’t generate immunity? After all, one case can be an accident, an aberration, an anomaly, an aardvark in a sea of anemone.

Well, oops something like this happened again, according to a more recent NHK-World Japan report. This time it was a man in his 70’s, who first tested positive for SARS-CoV2 on February 14 while on a Diamond Princess cruise ship. After being transferred to a medical facility in Tokyo, he stayed there until testing negative for the virus. On March 2, he left the facility and traveled home via public transportation. However, the man eventually began feeling sick with a fever, which prompted him to go to a hospital on March 13. The following day he tested positive for the virus again.

Then there’s the February 14 article from Caixin, a Beijing, China-based media group, that was entitled “14% of Recovered Covid-19 Patients in Guangdong Tested Positive Again.” Umm, 14% would seem more like an “ooop” than an “ooops.” This CGTN news warned of such reinfection possibilities:

Remember though, these are news reports and not scientific studies yet. While the reappearance of Nicholas Cage with a flaming skull riding a motorcycle may not call for additional scientific studies, all of these cases certainly do. First, scientists need to confirm whether the test results were indeed accurate. Remember, no test is perfect. If people can screw up a drink order, they can certainly mess up a medical test. Even if a test is performed properly, you could still get a positive result when you don’t actually have an infection. On the flip side, just because you test negative doesn’t necessarily mean that there is no way that you are carrying the virus. That’s why a doctor may test you multiple times to be sure of a result.

Secondly, doctors and other scientists need to double-check or triple-check that each of these patients actually got re-infected with the virus rather than had an infection that simply lasted a long time. What if, for example, the cruise passenger and the tour bus guide each had fairly long infections and just happened to have intervening false negative test results? The tests could have simply been like commercial breaks in the middle of a single long episode of a television show.

Third of all, the amount of immunity that you build up after being exposed to any virus depends on not only virus itself but surprise, surprise your immune system and its response. When your immune system sees a particular virus for the first time, it can essentially get caught with its pants down, not ready to defend your body against this new invader. However, exposure to the virus either through a vaccine or getting infected may train your immune system so that, borrowing the words of former President George W. Bush, “fool me once, shame on — shame on you. Fool me — you can’t get fooled again.” If strong enough, your immune system then may be ready with proper defenses next time the virus comes calling. Could the cases of reinfection then be examples of people who happened to have weaker immune systems?

Or are these cases any indication that our immune systems may not be able to consistently build up enough protection against SARS-CoV2? Well, a review article published in January 2020 in the Journal of Medical Virology summarizes much of what is known about your immune system’s response to various types of coronavirus. As you can see, this involves a complex orchestra of different cells and chemicals. Therefore, the immune response to one virus won’t necessarily be the same as to another virus, even if both viruses were different types of coronaviruses. All of this also depends on how strong your immune system may be and how well your immune system recognizes an invader like SARS-CoV2.

Plus, your immune system has got to remember the virus. Over time, immunity may fade, allowing the virus to reinfect you. It’s like when you get back together with an ex after you have forgotten how terrible you are for each other. The question then is how long can your immune system remember SARS-CoV2?

With SARS-CoV2 having emerged so quickly, there just haven’t been enough studies yet on how your immune system may react specifically to SARS-CoV2 and how this may differ from person to person. Therefore, we have to rely on studies of other coronaviruses for now. The closest approximation is probably the even more evil cousin of SARS-CoV2, the original SARS virus that caused the outbreak of 2002-2003.

In a study published in a 2007 issue of Emerging Infectious Diseases, a research team from the Shanxi Provincial Center for Disease Control and Prevention in Taiyuan, China, followed 176 patients who had had severe acute respiratory syndrome (SARS). On average, SARS-specific antibodies remained at the same level in a patient’s blood for about two years. Then, during the third year after infection, antibody levels tended to drop precipitously. This suggests that immunity to the SARS virus may remain for two to three years with reinfection possible after three years.

Keep in mind though that antibody levels do not always correlate with immunity. They can be like selfies on Instagram, only indirect measures of what’s really going on at a deeper level. Some people may have immunity against a virus without detectable antibody levels, and some people may be very susceptible to infection even though antibodies are present. The only way to have determined if the patients actually had immunity against the SARS virus would have been to have re-exposed them to the virus and checked what happened. And that would have been a horrible experiment to do.

The other question is how many different versions of SARS-CoV2 may be running around, or rather spreading around since viruses don’t have little feet and little sneakers. It’s difficult to answer this question for sure without more thorough and widespread testing. According to a study published in the journal National Science Review, an analysis of samples from 103 COVID-19 cases suggests that at least two different versions of SARS-CoV2 are circulating. This doesn’t necessarily mean that these versions are so different that immunity to one version doesn’t mean immunity to another. Regardless, things may evolve in the near future. Viruses can be like the characters in Game of Thrones or an actor in a Broadway show, changing rapidly. Over time, the new coronavirus could possibly mutate to the point that new versions are no longer as recognizable by your immune system as the original version. After all, mutations are probably what allowed the virus to jump from another animal to humans.

Not knowing exactly how immunity against SARS-CoV2 works and how long it may last throws a gigantic wrench into public health planning. Many trying to predict the course of the pandemic have been assuming that once a high enough proportion of the overall population has been infected and has become immune, the pandemic will subside. Herd immunity is the percentage of the overall population that is immune to a given pathogen. When this percentage gets high enough, the virus will struggle to find more susceptible people to infect, sort of like trying to sell Justin Bieber T-shirts in a crowd when most of the people are already wearing such shirts. The belief is that when around 70% of the population is immune to the virus, SARS-CoV2 will struggle to continue transmitting.

However, things could change substantially if people can actually get re-infected with the virus or different enough versions of the virus end up circulating. Such possibilities would be yet more reasons to question the “herd immunity” approach to controlling the pandemic that’s currently being discussed in the U.K. and described by Sarah Boseley for The Guardian. Since there is no vaccine available against SARS-CoV2, there is actually talk of allowing those with stronger immune systems to get infected to achieve the 70% or so herd immunity threshold. Huh?

This strategy would make sense except for the fact that it doesn’t. First of all, those who get infected could end up having serious consequences such as death, which is typically a very serious consequence. This would be reminiscent of the saying that “the operation was successful, but the patient died.” Allowing people to become infected by a potentially deadly virus is always a risky proposition, sort of like playing roulette when your lungs are on the betting table. So far, the COVID-19 case-fatality rate seems to be somewhere between 1% and 3.4%. This isn’t as high as the rate for SARS but nonetheless significantly higher than that of a bad flu season.

Secondly, this herd immunity strategy depends on people not getting re-infected with the virus. But with the aforementioned reports from Japan and China, you have to wonder if the strategy is not a “herd immunity” strategy but rather a “herd immunity maybe” strategy to borrow the words of Carly Rae Jepsen. “Maybe” may work to some degree with flirting and dating but not when lives are at stake.

Third of all, this strategy assumes that people will not leave or enter the U.K. That may work only if you want to completely eliminate travel to and from the country.

Finally, such a strategy would run counter to other mitigation strategies such as social distancing as indicated by the following tweet:

Uh, U.K., would this really be O.K.?

All of this is a reminder that scientists do not yet know enough about this new coronavirus. What percentage of people become immune to the virus if exposed? How strong is the immunity? Will it actually prevent reinfection? How long would this immunity last? Is it two years as the SARS study hints at or could it be much shorter than that? How does all of this vary from person to person? How many different versions of the virus may end up circulating? As the Internet meme goes, I and many other scientists have so many questions.

Therefore, if you do get exposed to the virus and recover, don’t view it as a free pass to start hugging strangers, digging your fingers deep into your nose like you are looking for pocket change, and licking door knobs. Keep doing what everyone else should be doing such as social distancing, washing your hands frequently and thoroughly, keeping your filthy fingers from gravitating towards your gigantic face, and actively disinfecting surfaces, objects, and that enormous BTS statue that you have in your living room. Just because you survived the first infection, doesn’t necessarily mean that future exposures and possible infections will end up OK. As you know, sequels don’t always have the same endings.