Swab being collected for a coronavirus test in Delhi. (Express file photo by Amit Mehra)

Just about two weeks ago, the Centers for Disease Control (CDC) in the United States had said that, until then, no case of reinfection with the novel coronavirus had been confirmed in any individual. The CDC did acknowledge that there had been instances in which people had tested positive again after having recovered from the disease once. But that, it said, could be because of what it described as “persistent viral shedding”. Recovered patients can have low levels of virus for up to three months after the infection, and though it is no longer enough to make the person sick or transmit the disease to others, it can get detected in diagnostic tests.

Now, it appears, the CDC may have spoken a little too soon. Researchers in Hong Kong recently reported the first known case of reinfection. A 33-year-old Chinese was found carrying the disease four-and-a-half months after recovering from the first infection. The patient had travelled to Spain in between. Just a day later, one similar case of re-infection was reported from Belgium and another from the Netherlands. And last week, the United States too reported its first case of presumed reinfection.

These first cases of reinfection show that the immunity developed after contracting the disease once may not be long-lasting, and the virus could continue to circulate in the populations even after levels of community-wide immunity, either through natural infection or by vaccination, are reached. It has also triggered concerns about the effectiveness of vaccines being developed for the infection.

But are these reinfections real?

Based on the paper that has been published by the Hong Kong researchers, scientists agree that the case of the 33-year-old Chinese person is indeed one of reinfection, and not of traces of virus from the previous infection getting detected again. In fact, the case for reinfection is not being made on the basis of results of a diagnostic tests. It so happened that the viral specimen from this person was collected both times he got infected, and genome sequences from the two specimen have been extracted, and compared.

The genome sequence analysis shows that the person had been infected with two different variants of the novel coronavirus. Viruses mutate with time, developing small differences in their genetic make-up. This enables researchers to trace the origins of the infection, and the possible routes through which it has been circulating. In this person, the variant of virus extracted the second time resembled closely to those in circulation in Europe in July and August. That could possibly mean that he had been re-infected during his travel to Spain. He had tested positive while being screened at the airport on his return.

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In the case of the reinfection in the US too, researchers in Nevada found that the virus associated with each instance of the patient’s infection represented genetically different strains.

What are the implications of these findings?

Scientists say the fact that reinfection has been confirmed does not come as a surprise. In fact, many believed that reinfection might have been happening earlier as well. It was just that it was not getting detected. That is because in none of the cases where a person had tested positive for the second time was a genome sequence from the first infection available. Genome sequences are not obtained from every coronavirus patient. Researchers extract the viral specimens from a few hundred patients for carrying out their studies. In the case of both the Hong Kong and the United States reinfections, researchers were lucky that they had the respective viral specimens collected the first time as well.

But other viruses, including those from the coronavirus family, especially those that cause respiratory illnesses, infect a large number of people every year, and those who have been infected once are not considered immune from them. As the Hong Kong researchers themselves point out, reinfection is common for these other seasonal coronaviruses. There is no reason to believe why SARS-CoV2, the virus that causes Covid-19 disease, would be any different. The difference, of course, is that unlike other viruses, there is no treatment available for Covid-19 right now.
But these first cases of re-infection do not mean that everyone is equally susceptible to reinfection. Who all can get re-infected, and how long an infected person can expect to remain protected from a reinfection, are both open questions right now that need further investigation. The Hong Kong researchers noted that in the Chinese patent, the disease was be milder in reinfection as compared to the first infection.

Would vaccines be effective?

Shahid Jameel, a virologist, believes these cases of reinfection do not raise any fresh question marks over the effectiveness of the vaccines being developed. That is because a reinfection would happen only from the different variants of the virus that are currently in circulation, and most of the vaccines being developed are meant to provide protection against all the variants of the virus. For blocking the virus, they are targeting regions that are common in all these variants. “Effectiveness of the vaccines because of the possibility of re-infection does not seem to be a concern right now,” he said.

“Of course, once these vaccines are in use, it is possible that the virus is forced to mutate in new ways because of natural selection pressures. In that case, the virus may change in a form that cannot be targeted with present vaccines. But that is in the future,” he said.

But as in the case of natural immunity, the longevity of protection offered by vaccine remains an open question. The Hong Kong researchers do point out that vaccines may not be able to provide lifelong protection against Covid-19 disease. But there are vaccines for several other diseases as well that need to be repeated periodically because they do not offer permanent protection.

The researchers, however, say that there is now a case for testing the vaccines being developed on recovered patients as well.

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