Hope for COVID-19 herd immunity has come under threat as a new study released suggests that those that test positive for coronavirus but never develop symptoms, asymptomatic carriers, have few to no antibodies shortly after infection, suggesting that lasting immunity may be unlikely.
Herd immunity is a term that refers to a large proportion of a population that has built immunity to a contagious disease, preventing it from spreading to even those who are not immune.
Asymptomatic carriers of the virus have been a key problem for public health authorities since the onset of the pandemic as it has proven difficult to identify everyone who has become infected and prevent COVID-19 from spreading.
The study, published in science journal Nature earlier this month, followed 178 laboratory-confirmed coronavirus patients. Of these, just over one in five, 20.8 percent, never developed any symptoms.
In comparing 37 that were asymptomatic and 37 that did show symptoms, the researchers found that the level of antibodies decreased across the board eight weeks after they were discharged from the hospital.
The study found that 12.9 percent of patients who had shown symptoms had no detectable antibodies after this period, but among asymptomatic carriers this number was far higher at 40 percent, suggesting antibodies are lose faster in those that show no symptoms than those that did.
Furthermore, asymptomatic carriers also tested positive for the virus for an average of five days longer than those with no symptoms did, at 19 days compared to 14 days, suggesting that asymptomatic carriers shed the virus for longer and could remain infectious for longer.
As such, the researchers warn that authorities will need to be cautious in easing lockdown measures as immunity may not last as long as hoped – particularly among those that show no symptoms.
“Together, these data might indicate the risks of using COVID-19 ‘immunity passports’ and support the prolongation of public health interventions, including social distancing, hygiene, isolation of high-risk groups and widespread testing,” the researchers wrote.
The researchers warned that the percentage of those showing no infection in the study was not likely to be an accurate estimate for how many asymptomatic carriers there are in a population and that more testing is needed.
The controversial strategy
A herd immunity strategy has been controversially used in Sweden, where a business-as-usual approach without imposing social distancing measures or curfews was adopted in the face of the pandemic – in contrast with broad global lockdowns.
However, this research suggests populations may not develop immunity fast enough to prevent the virus reinfecting those that have already recovered from the virus, dashing hopes of herd immunity stopping the virus in its tracks.
Furthermore, there is yet to be enough research conducted into the relationship between antibodies and immunity. Scientists have still not conclusively determined how long antibodies last, and whether the presence of coronavirus antibodies are necessary to maintain immunity.
In an interview with Canadian broadcaster CBC, Dr. Samir Gupta, a clinician-scientist and researcher in Toronto, said that it is not surprising that antibody levels fell some time after infection.
Furthermore, Gupta added that the body had other methods of fighting the virus, such as memory cells, that are harder to detect but allow the body to begin releasing antibodies when it encounters the same virus again, potentially providing a reason to hope for a herd immunity strategy.