Coronavirus patients not showing symptoms may have weaker immune responses to disease: Study

Asymptomatic patients shed the virus for a longer period as compared to those who have symptoms. Study findings suggest that there are potential risks of using Covid-19 ‘immunity passports’


                            Coronavirus patients not showing symptoms may have weaker immune responses to disease: Study
(Getty Images)

People who have Covid-19 but do not show any symptoms of the disease may have a weaker immune response to the virus. Researchers from various institutions in China found that asymptomatic people also shed the virus when infected - implying that they may be infectious and may spread coronavirus to others and they did so for a longer duration than those who have symptoms. 

The research team studied 37 asymptomatic individuals in the Wanzhou District who were diagnosed with confirmed SARS-CoV-2 (the virus that causes Covid-19) infections but without any relevant clinical symptoms in the preceding 14 days and during hospitalization. Of the 37 asymptomatic individuals, the median age was 41 years (range, 8-75 years) and 22 were female. Their health data were compared with 37 other Covid-19 patients who had mild symptoms. Researchers found that asymptomatic patients had levels of antibodies that were significantly lower than what was found among patients who had symptoms, which suggests the asymptomatic patients had weaker immune responses.

Antibodies are proteins that help fight off infections. Antibody tests or serologic tests use blood samples to see if a person’s immune system has created antibodies to fight Covid-19. They allow doctors to understand if a person has been exposed to the virus in the past. They test for IgG, IgM, and IgG or total antibody. In the current study, to investigate the acute antibody response to SARS-CoV-2 infection, virus-specific IgG and IgM were measured in serum samples from asymptomatic and symptomatic individuals.

According to the analysis, the levels of virus-specific immune system molecules called IgG antibodies were significantly lower in the asymptomatic group than in the symptomatic group during the acute phase of infection when the virus could be detected in the respiratory tract. Eight weeks after the patients were discharged from the hospital, the levels of neutralizing antibodies decreased in 81% of asymptomatic patients, compared to 62% of symptomatic patients.

“In the asymptomatic group, 81.1% (30/37) tested positive for IgG, and 83.8% (31/37) of the symptomatic group tested positive for IgG approximately 3-4 weeks after exposure. Moreover, 62.2% (23/37) of the asymptomatic group were positive for IgM, whereas 78.4% (29/37) of the symptomatic group were IgM positive. Interestingly, IgG levels in the symptomatic group were significantly higher than those in the asymptomatic group in the acute phase (the period when the viral RNA can be found in a respiratory specimen,” say researchers in the study published in Nature Medicine.

“We also followed 37 asymptomatic individuals and 37 symptomatic patients into the early convalescent phase (8 weeks after they were discharged from the hospital). The IgG levels in the symptomatic group were still significantly higher than those in the asymptomatic group in the early convalescent phase. Surprisingly, the IgG levels in 93.3% (28/30) of the asymptomatic group and 96.8% (30/31) of the symptomatic group declined during the early convalescent phase. We also observed a decrease in neutralizing serum antibodies levels in 81.1% (30/37) of the asymptomatic group and in 62.2% (23/37) of the symptomatic group. Besides, asymptomatic individuals exhibited lower levels of 18 pro- and anti-inflammatory cytokines. These data suggest that asymptomatic individuals had a weaker immune response to SARS-CoV-2 infection,” they explain. 

Researchers found that asymptomatic patients had levels of antibodies that were significantly lower than what was found among patients who had symptoms. (Getty Images)

The study attempted to answer another critical question: how long do the antibodies last. They found that antibodies may last only two to three months, especially in people who never showed symptoms while they were infected. “We observed that IgG levels and neutralizing antibodies in a high proportion of individuals who recovered from SARS-CoV-2 infection start to decrease within 2-3 months after infection,” say the team. The strength and duration of immunity after infection are key issues for ‘shield immunity’ and for informing decisions on how and when to ease physical distancing restrictions According to the researchers, previous studies have shown that circulating antibodies against SARS-CoV or MERS-CoV lasted for at least 1 year. “Sustained IgG levels were maintained for more than 2 years after SARS-CoV infection. Antibody responses in individuals with laboratory-confirmed MERS-CoV infection lasted for at least 34 months after the outbreak,” they add.

The analysis also shows that the average duration of viral shedding in 37 patients with mild symptoms were 14 days. In comparison to symptomatic patients, the asymptomatic group had a significantly longer duration of viral shedding of 19 days. “Several factors might contribute to the variation of duration of viral shedding in different studies, including the severity of the disease, definition of duration of viral shedding, and frequency of specimen collection. Notably, detection of viral RNA does not necessarily mean that infectious virus is present in respiratory specimens, and caution is required when applying virus shedding duration that was calculated based on RT–PCR to assess infection potential,” says the team. 

Overall, the researchers said that their findings suggest that there are potential risks of using Covid-19 ‘immunity passports’ to certify a person has recovered from Covid-19 and “support the prolongation of public health interventions,” including social distancing, hygiene, isolation of high-risk groups and widespread testing. The World Health Organization (WHO) had earlier said that there is currently “no evidence” that people who have recovered from Covid-19 and have antibodies are protected from a second coronavirus infection. Accordingly, the WHO warned countries against issuing a “risk-free certificate” or “immunity passport” to such people on the assumption they would be immune to reinfection.

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