NIH study confirms SARS-CoV-2 reinfections are relatively rare
A new study examining data from more than three million people suggests reinfection with SARS-CoV-2, the virus that causes COVID-19, is still quite rare. The research affirms a positive antibody test following an initial infection can be associated with a significantly lower risk of a second infection in the future, however, it is still unclear how long this protection may last.
After the initial big wave of the pandemic passed in the first half of 2020, scientists started closely monitoring infection data to understand how likely people were to catch the virus a second time. Adding some noise to the data was the fact that our gold-standard test for SARS-CoV-2 (called a polymerase chain reaction, or PCR, test) is incredibly sensitive to viral fragments. COVID-19 patients can test positive on PCR tests months after recovering due to persistent shedding of viral RNA.
In August last year scientists from the University of Hong Kong reported the first clinically confirmed case of SARS-CoV-2 reinfection. Because this particular case offered genomic data from both infections, the researchers could positively confirm the second infection was a different strain of the virus and not prolonged viral shedding.
Now, over a year into the pandemic, there are still unanswered questions regarding the duration of immunity following an initial infection. Many researchers have tried to find an answer by measuring levels of immune antibodies in patients following an initial infection.
Several of these antibody studies have concluded levels can drop rapidly in the months following an initial infection. But this specific metric cannot directly equate to a reinfection risk. Our immune system has many tactics to fight against infection and some researchers suggest longer-term immunity may not be effectively measured by simply tracking antibody levels.
This new study, led by researchers from the National Cancer Institute, a division of the National Institutes of Health, looked at anonymized real-world data encompassing millions of subjects. Data from more than three million SARS-CoV-2 antibody tests were analyzed. Around 12 percent of those antibody tests were found to be positive.
The researchers then looked at how many of those millions of subjects presented with a positive PCR test for SARS-CoV-2 in the months following an initial antibody test. Examining the data the researchers saw PCR positivity rates declining in those subjects who initially tested positive in the antibody test.
This suggests a period of viral shedding following an initial infection can lead to persistent positive PCR tests. But, positive PCR tests for SARS-CoV-2 do certainly decline after several months, affirming reinfection is uncommon.
“The data from this study suggest that people who have a positive result from a commercial antibody test appear to have substantial immunity to SARS-CoV-2, which means they may be at lower risk for future infection,” says Lynne Penberthy, lead on the new research. “Additional research is needed to understand how long this protection lasts, who may have limited protection, and how patient characteristics, such as comorbid conditions, may impact protection. We are nevertheless encouraged by this early finding.”
The results mirror a recent UK study tracking more than 20,000 health care workers in the United Kingdom. That research, still in pre-print and not yet published, suggests individuals with a prior history of SARS-CoV-2 infection are 83 percent less likely to be infected a second time. The median protective period covered by the UK study was five months.
Douglas Lowy, an author on the new study from the National Cancer Institute, is cautious to note his findings can only really suggest a natural SARS-CoV-2 infection may be linked to partial immunity against reinfection. He says recovered COVID-19 patients should still get vaccinated as it is unclear how long any natural immunity may last. Plus, the new research does not take into account potential reinfection rates from newer variants of the virus.
"The results from the study are basically a 10-fold reduction,” Lowy said in an interview with CNN, “but I would have caveats around that. In other words, it could be an overestimate of the reduction. It could be an underestimate of the reduction.”
The new study was published in the journal JAMA Internal Medicine.