Doctors in Hong Kong say they have hard genetic evidence that a 33-year-old man has been reinfected with SARS-Cov-2, the coronavirus that causes Covid-19, over 4.5 months after his first bout.
Independent researchers commenting on the report say the evidence is both worrying and compelling as it raises some important questions about immunity to Covid-19. However, they stress people should not be jumping to conclusions as this remains a one-off observation for now.
The findings by the University of Hong Kong were announced on Monday and are due to be published in the journal Clinical Infectious Diseases this week. A draft of the report can be found here.
If accurate, it would be the world's first proven case of reinfection. Researchers have previously reported dozens of cases where people have recovered from Covid-19 but since tested positive for the disease again. However, it remained unclear whether this was a technical issue with the tests and sampling. This new report is the first to use genetic analysis of the virus to confirm that reinfection can occur.
As the report explains, a 33-year-old seemingly healthy man caught his initial infection of SARS-CoV-2 in late March. He was hospitalized on March 29, but was discharged two weeks later after his symptoms disappeared and he had tested negative twice. On August 15, some 4.5 months later, he became infected once again. After returning home from Spain via the UK, he was tested for SARS-CoV-2 on arrival at Hong Kong airport and received a positive result. However, he did not feel ill during this second infection and remained asymptomatic. Genetic analysis of the viruses showed that the patient was infected by two distinct strains of SARS-CoV-2.
The findings could have some implications for science’s understanding of immunity to SARS-Cov-2. For starters, it suggests that immunity to Covid-19 is not permanent. As other research has suggested, antibodies to SARS-Cov-2 may fade within two to three months in some cases. This indicates that it’s very unlikely “herd immunity” can eliminate SARS-CoV-2. It also raises some doubt over whether vaccinations for Covid-19 will provide meaningful long-term protection.
“This is a worrying finding for several reasons. The first... is that it suggests that previous infection is not protective. The second is that it raises the possibility that vaccinations may not provide the hope that we have been waiting for," commented Dr David Strain, clinical senior lecturer at the University of Exeter and chair of the British Medical Association’s Medical Academic Staff Committee.
In the instance of this patient, it’s safe to suggest that his second infection was made more likely because he had encountered a different strain of the virus, which his body had not acquired full immunity to.
“This is certainly stronger evidence of re-infection than some of the previous reports because it uses the genome sequence of the virus to separate the two infections. It seems much more likely that this patient has two distinct infections than a single infection followed by a relapse due to the number of genetic differences between the two sequences,” explains Dr Jeffrey Barrett FMedSci, a senior scientific consultant for the COVID-19 Genome Project at the Wellcome Sanger Institute, who was not involved in the study.
It’s also noteworthy that his second infection was asymptomatic. Infectious disease experts say this could suggest the first infection provided some degree of residual immunity, which helped to reduce the severity of the illness. Alternatively, his second infection may have involved a lower viral load or another factor that affects the nature of the symptoms experienced.
In short, the new report raises more questions than it answers. However, it should be noted that reinfections currently appear to be incredibly rare.
“Given the number of global infections to date, seeing one case of reinfection is not that surprising even if it is a very rare occurrence,” said Dr Barrett. “This may be very rare, and it may be that second infections, when they do occur, are not serious."