A man in Hong Kong who recovered from COVID-19 was reinfected with the virus four-and-a half months later in the first recorded instance of a second infection, researchers at the University of Hong Kong said on Monday.
The findings suggest the disease can infect multiple times despite herd immunity.
According to the researchers, a genetic analysis showed that these two successive infections of the same patient had been caused by two different strains of the SARS-CoV-2 virus, responsible for COVID-19.
The 33-year-old man was first diagnosed with COVID-19 in late March but tested positive again while being tested after he returned to Hong Kong from Spain via Britain on August 15.
But he was found to be infected with a different coronavirus strain the second time around and was asymptomatic.
The two viral signatures were said to be "completely different", and belonged to different coronavirus lineages, or clades.
The first closely resembled strains collected in March and April, and the second strain matched the virus found in Europe -- where the patient had just been visiting -- in July and August.
"Our study proves that immunity for COVID infection is not lifelong -- in fact, reinfection can occur quite quickly," said Kelvin Kai-Wang To, a microbiologist at Hong Kong University's Faculty of Medicine and lead author of a forthcoming study that details the findings.
"COVID-19 patients should not assume after they recover that they won't get infected again," he told AFP.
To also said those who have overcome the virus should still practice social distancing, wear masks and practise hand washing.
Experts have voiced different opinions on how alarmed people should be at the new findings, which will be published in the peer-reviewed medical journal Clinical Infectious Diseases.
"This is a worrying finding for two reasons," said David Strain, a clinical senior lecturer at the University of Exeter Medical School.
"It suggests that previous infections are not protective. It also raises the possibility that vaccinations may not provide the hope that we have been waiting for."
If antibodies don't provide lasting protection, "we will need to revert to a strategy of viral near-elimination in order to return to a normal life", he said.
But others say the new case is likely to be rare. .
"It is to be expected that the virus will naturally mutate over time," said microbiologist Brendan Wren of the London School of Hygiene & Tropical Medicine.
"This is a very rare example of re-infection and it should not negate the global drive to develop COVID-19 vaccines."
The virus has killed over 800,000 people worldwide and there has been an uptick of new infections after many countries lifted lockdown measures.