Asymptomatic Covid-19 patients may have weaker immune responses to infection, new study finds

People who have coronavirus infections but never develop symptoms could have weaker immune responses to the virus, a new study suggests.

The small study, published in the journal Nature Medicine on Thursday, found that a group of about three dozen Covid-19 patients who were asymptomatic had levels of antibodies that were significantly lower than what was found among patients who had mild symptoms — a finding that suggests the asymptomatic patients had weaker immune responses, reports the CNN.

The researchers, from various institutions in Chongqing, China, also found that the asymptomatic patients had a significantly longer duration of viral shedding — in which they could spread the coronavirus to others - than the symptomatic patients.

The new study included data on 37 Covid-19 patients who were diagnosed before April 10 and developed no symptoms while isolated at a hospital in the Wanzhou District of Chongqing, China. Their health data, taken from blood samples and other tests, were compared with 37 other Covid-19 patients who had mild symptoms.

The data showed that, even though the asymptomatic patients were experiencing no symptoms, they were still shedding the coronavirus — meaning they were infectious — for a median duration of 19 days.

That duration of viral shedding was significantly longer than what was found among the patients with mild symptoms, which was 14 days, according to the study.

The data also suggested that certain antibody levels among the asymptomatic patients were significantly lower relative to the symptomatic patients.

Antibodies, which are proteins that circulate in blood to help fight off infections, can hold clues to body's immune response.

Among the asymptomatic patients, 81.1 percent had reductions in their neutralizing antibody levels during the eight weeks after being discharged from the hospital — compared with 62.2 percent of the symptomatic patients.

The researchers found some other differences in the patients' health data, suggesting that "asymptomatic individuals had a weaker immune response" to the coronavirus. 

The study had some limitations, including that it involved a small number of patients and antibody testing itself is not always 100 percent accurate.

But overall, the researchers wrote in their study that the findings suggest there are potential risks to using "immunity passports" based on antibodies and "support the prolongation of public health interventions, including social distancing, hygiene, isolation of high-risk groups and widespread testing."

Eleanor Riley, a professor of immunology and infectious disease at the University of Edinburgh in Scotland, said that the new study's findings are not surprising.

"The data are in line with several recently reported studies suggesting that those with mild or asymptomatic infections make a less robust antibody response to SARS-CoV-2 than those with more severe disease," Riley said in a written statement distributed by the UK-based Science Media Centre on Thursday.

"The really interesting question, to which we don't yet have an answer, is why some people develop such mild infections," Riley said in part.

"It may be that they are genetically less susceptible to infection or that they have some pre-existing immunity due to prior infection with related seasonal coronaviruses."