A man returning from Europe spotted at Hongkong Airport tested Covid-19 positive for the second time in six months. Sequence analysis shows the strain of the virus, however, is different from the first time.
A 33-year-old man, returning from Europe to Hongkong on 15th August, was tested Covid-19 positive with no symptoms. SARS-CoV-2, the virus that causes Covid-19 also infected him in March. The researchers at Hongkong University (HKU) had performed genetic analysis of the virus the first time; they sequenced the virus again.
Comparing the first sequence analysis results with the second one, the researchers found that the second strain contained 24 nucleotide differences—suggesting the virus strain man infected with for the second time is different from the first one.
The result fueled the speculation that some patients who recovered from the infection may not have long-term immunity against the virus.
The report is accepted for publication in the journal Clinical Infectious Diseases. There have been several reports of Covid-19 patients testing positive after getting clearance from their first-time infections. First, such news surfaced in Mar 2020 in South China Morning Post that says a 38-year-old man was released from a makeshift hospital in Wuhan China after testing Covid-19 negative twice.
Then, within a week he was admitted to the hospital with the symptoms again and died shortly after the admission. Similar cases like discharged patients readmitting to the hospital after falling ill again have been reported within the same hospital and elsewhere.
Many of these types of cases can be seen as an error from sample collection or testing. Perhaps the patients were not quietly free from the virus at the time of their release from the hospital. It is also possible the released patients were not fully recovered from the damage the infection caused to their vital organs: lung, kidney, or heart.
Even more, the discharged patient probably did not develop enough neutralizing antibodies required to give a long-term shield against the virus during their first infection.
In the Hongkong case, for example, the man tested negative for the antibodies IgG, a vital part of the immune response, even after ten days of developing symptoms in the first infection. Therefore, the authors of the report write, “People with low neutralizing antibody titers will be expected to be more susceptible to reinfection.” It is unlikely that herd immunity can eliminate SARS-CoV-2, and vaccines may not be able to provide life-long protection against Covid-19 the authors write in their paper.
It is not, however, clear how different virus have to be to reinfect a patient, but the study suggests that “Covid-19 patients should not be complacent about prevention measures” and should continue social distancing, wearing masks and other ways to reduce infection, said Dr Kelvin Kai-Wang To, the microbiologist who led the work. “if immunity wanes from natural infection, it could be a challenge for vaccines” and may mean booster shots are needed, he added.
Other experts say it is necessary to relook at the occurrence of similar cases on a population level as there are over 16 million recovered cases to date.
Many think this case should not be considered as an isolated one as he was spotted at the airport, in an unusual situation. There may be many more such cases around who are asymptomatic and unaware if they are infected again.
The research questions, will, however, remain: how often this happens, if reinfections are more likely asymptomatic, will they able to infect others, will it trigger serious illness, or will it occur in a vaccinated case?