New research has suggested that antibodies to Covid-19 may fade away within two to three months among people who were infected but didn’t experience any symptoms, indicating that any substantial long-term immunity is unlikely for those people. However, this does not necessarily mean that these people can be infected a second time, at least in the short term.
While very little is still known about Covid-19 immunity, the study could have some implications for how the world approaches the ongoing pandemic in the future, such as the controversial plans to roll out “immunity passports”.
Reporting in the journal Nature Medicine, scientists from Chongqing Medical University in China compared the immune response to SARS-CoV-2, the virus that causes Covid-19, between asymptomatic people and symptomatic people in the city of Chongqing between January and February of this year.
During the illness, levels of virus-specific antibodies were found to be significantly lower in the asymptomatic group than in the symptomatic group, suggesting people without symptoms launched less of an immune response. They also discovered that viral shedding — when the body releases the virus, ready to potentially infect others — occurred for 19 days in asymptomatic people, compared with 14 days in the symptomatic patients. This suggests some asymptomatic people could be potentially infectious for slightly longer than others.
Eight weeks after the illness had resolved, levels of neutralizing antibodies decreased by 81 percent in asymptomatic patients, compared to 62 percent in symptomatic patients. Asymptomatic patients also had lower levels of 18 pro- and anti-inflammatory cytokines, cell-signaling proteins, which further indicates a weaker immune response.
Experts who were not directly involved in the study have said these findings are “not surprising” and are largely in line with what’s already known about mild infections of any cause, although the research has raised concern among some scientists.
"This strongly suggests that immunity may well diminish within months of infection for a substantial proportion of people," commented Professor Liam Smeeth, Professor of Clinical Epidemiology at the London School of Hygiene and Tropical Medicine.
"We need larger studies with longer follow-up in more populations, but these findings do suggest that we cannot rely on people having had proven infections nor on antibody testing as strong evidence of long term immunity,” he added.
Some governments have toyed with the idea of using “immunity passports” that would allow people who can prove they’ve already been infected with Covid-19 to travel more freely and access more public areas. This research, however, suggests the plan is not foolproof as it’s wrong to assume everyone who's had Covid-19 has picked up solid long-term immunity to the disease.
That said, there is some hope that even asymptomatic patients might be protected from a second infection, at least in the short term. Another study published in Nature last week found that some neutralizing antibodies found among asymptomatic people in lower levels are actually some of the most powerful. This hints that the lower levels of antibodies will still protect you from getting sick again in the short term, although the bigger question of long-term immunity remains unanswered just yet.