Protective immune responses that build up during a SARS-CoV-2 infection may weaken just two to three months later, especially if the infection showed no symptoms, a new study suggests.
The finding does not necessarily mean that people are no longer immune to the new coronavirus after a few months. The lower levels of immune responses measured in the study may still be enough to thwart the virus, and there are other types of immune responses not studied in the study that play a role in immunity. Overall, there are still many unknowns about potential immunity to SAR-CoV-2 infections, including who is most protected and how long that protection can last.
But the authors of the new study say their findings are enough to raise concerns about the possible use of so-called “immunity passports” — documents that indicate someone is immune based on a previous infection. The authors – a team of researchers in Chongqing, China – also suggest that their findings support the continued use of physical distancing and other prevention efforts until we have a better understanding of immunity.
“The strength and duration of immunity after infection are important issues for ‘shield immunity’ and for informing decisions about how and when physical distancing restrictions can be relaxed,” the writing writes. Their study appeared Thursday in Nature Medicine.
For the study, the team looked at immune responses in asymptomatic and symptomatic COVID-19 patients shortly after their infection and then two to three months later. The study was small and included just 37 asymptomatic cases, but it’s one of the first to look at detailed immune responses in people who never develop symptoms during their infection.
The asymptomatic cases were identified by contact tracing of known cases and then isolated in a hospital for the entirety of their infection. The 37 were identified from a total of 178 cases in Chongqing’s Wanzhou district, for an asymptomatic case count of 21 percent. Although no one developed noticeable symptoms, more than half had abnormalities in their breast scans during their infection.
The researchers compared their immune response to that of 37 people who had cases of symptomatic COVID-19. These symptomatic controls were compared to the asymptomatic cases based on gender, age and underlying health conditions.
Asymptomatic cases secrete viral genetic material from their throats longer than the symptomatic cases – a median of 19 days compared to 14 days, respectively. However, the presence of viral genetic material does not necessarily reflect infectious viral particles, so it is unclear whether asymptomatic cases were infectious for longer than symptomatic cases.
About three to four weeks after the first exposure of each case, the researchers looked for antibodies, that is, Y-shaped proteins the immune system makes to identify and disarm germs the body has already encountered. In general, the asymptomatic cases had lower levels of anti-coronavirus antibodies than their symptomatic counterparts. They also had lower levels of inflammatory immune signals.
When the researchers looked at antibody levels again eight weeks after each case was discharged from the hospital, they found that both groups had significant decreases in antibodies. In the asymptomatic group, 40 percent had no detectable levels of one type of antibody — IgG — while 13 percent of the symptomatic cases had no detectable levels. In comparison, in people infected with SARS-CoV-2’s relative, SARS-CoV (the coronavirus that causes SARS), researchers have seen sustained IgG levels for more than two years.
But not all antibodies are equally effective at counteracting invading viruses, such as SARS-CoV-2. The most effective are called neutralizing antibodies. The researchers of the new study specifically looked for these antibodies using an engineered pseudovirus designed to mimic SARS-CoV-2 as bait. The researchers found that eight weeks after recovery, neutralizing antibody levels were still present, but they had decreased in 81 percent of asymptomatic cases and 62 percent of symptomatic cases.
What this means for immunity is not yet clear. Another study published Thursday in Nature found that some neutralizing antibodies present in low levels are also the most potent — possibly an indication that low levels may be enough to protect against infection. But researchers don’t know that yet.
The authors of the Nature Medicine study say more antibody studies “profiling more symptomatic and asymptomatic individuals are urgently needed to determine the duration of antibody-mediated immunity.”
naturopathy2020. DOI: 10.1038/s41591-020-0965-6 (About DOIs).