SINGAPORE - News that a 33-year-old man in Hong Kong was reinfected with Covid-19 has sparked fears that people can catch the disease more than once, but infectious disease experts said this is rare and not a cause for worry, unless many more cases surface.
Most people develop a robust immune response after a Covid-19 infection, but for reasons that are currently unknown, the man in Hong Kong had failed to mount an antibody response to the first infection in late March.
He was hospitalised and discharged on April 14 after two negative swabs from his nasopharynx and throat. Then, he tested positive for Covid-19 on a saliva test on Aug 15, after returning from a trip to Spain via Britain.
Researchers at the University of Hong Kong (HKU) said they found the world's first documented case of reinfection, after they sequenced the virus from the man's infections and found that he had a different strain from the one in March.
Professor Paul Tambyah, a senior infectious diseases consultant at the National University Hospital, said: "The likely reason he was reinfected with a different strain is that he had no detectable antibodies after the first infection - possibly because it was so mild (he was asymptomatic throughout his hospitalisation) but also possibly because of some uncommon specific immune defect."
This appears to be a rare case, "just like we occasionally get people who have two episodes of chicken pox or even measles", he added.
Globally, there are more than 23 million cases, and before the Hong Kong case, there have been only some anecdotal reports of people being reinfected with Covid-19.
Interestingly, the Hong Kong man had no symptoms the second time around, while during his first infection, the symptoms were mild.
Prof Tambyah said the first infection seems to have protected the man from a serious infection during the second exposure. It also primed him for a late antibody response when exposed to the virus again, even though it was a different strain.
On Twitter, Prof Akiko Iwasaki, an immunologist at Yale University, said that the man's immunity had protected him from disease, even if it was not enough to block reinfection.
She said it is encouraging to note that while the patient had no detectable antibodies at the time of reinfection, he developed detectable antibodies after the reinfection.
She also said that since reinfection can occur, herd immunity by natural infection is unlikely to eliminate the virus that causes Covid-19. The only safe and effective way to achieve herd immunity is through vaccination.
The HKU researchers reportedly said the reinfection case would mean those who have been infected with Covid-19 should still be offered vaccination, and comply with mask-wearing and social distancing restrictions.
Their study is reportedly due for publication in the American journal Clinical Infectious Diseases.
"This is a single case, but represents something we felt was plausible - that immunity post-infection may not be lifelong, just like for other human coronaviruses or influenza," said Associate Professor Hsu Li Yang, an infectious diseases specialist from NUS' Saw Swee Hock School of Public Health.
Vaccine-conferred immunity may not be lifelong as well, but it may perhaps be more like the influenza vaccine, in which repeated vaccinations are required, he added.
Still, Prof Ooi Eng Eong, deputy director of Duke-NUS Medical School's emerging infectious diseases programme, pointed out that the immune response to vaccination is different from that against a Sars-CoV-2 infection.
"The virus contains genetic elements that are able to suppress our immune response, whereas vaccines are designed to stimulate these responses," he said.
"The resultant immunity from vaccination and infection would likely be different."
There are still a lot of unknowns about the Hong Kong case. For instance, it is not known if the reinfected man was able to mount a T-cell response to the initial infection, even if he did not have an antibody response, said Prof Tambyah.
He added that knowing this will lead to a better understanding of the level of immunity a vaccine needs to achieve in order to not just prevent severe disease but also prevent asymptomatic infection.
T-cells are a critical part of the immune system that also help us fight the disease.
In July, a study done by Prof Antonio Bertoletti at Duke-NUS' emerging infectious diseases programme and his team, had suggested that T-cells may offer long-lasting immunity against Covid-19.
The findings, reported in the journal Nature, showed that patients who recovered from Sars 17 years ago, after the 2003 outbreak, still had memory T-cells, which recognised parts of Sars-Cov-2, the virus that causes Covid-19.
The team also tested people who were not infected with Covid-19 and found the Sars-Cov-2 specific T-cells in more than half of them, which suggest that a level of pre-existing Sars-Cov-2 immunity is present in the general population.
In a recent Facebook post reproduced on the HKU website, clinical assistant professor Siddharth Sridhar from HKU's department of microbiology said that for most individuals, a Covid-19 infection would confer a robust immune response. This response is likely to be protective against severe symptomatic reinfection, at least in the short term.
"Some asymptomatic or mildly infected Covid-19 patients have weak antibody responses that decline rapidly, but they still mount a strong 'T-cell' response," he said.
"Our amazing immune systems will adapt to this virus and deal with it (hopefully with a bit of help from vaccines)," he said.