One of the mysteries about SARS-CoV-2 is why some patients, even those with no underlying health conditions, become critically ill or die, while others are asymptomatic, even patients classed as high risk. Infection with SARS-CoV-2 can produced a very wide spectrum of symptoms. While the majority of patients infected with SARS-CoV-2 will experience a mild to severe respiratory illness 2-14 days after exposure to the virus there are patients who will not display any symptoms.
The reason why some patients appear to be unaffected by the virus is not well understood, and many studies are currently underway to identify why this may be the case, as it could hold the key to developing a cure for the disease and identifying which individuals are likely to be most – or least – at risk.
There have been several hypotheses proposed as to why some people are unaffected yet the disease is fatal in other patients. It has been proposed the severity of COVID-19 may be due to the amount of virus an individual is exposed to, with a higher exposure leading to a much more severe form of COVID-19. There is one train of thought that suggests previous infection with a different coronavirus may provide a degree of immunity, or that the severity of the disease is linked to the immune response of the patient. There is little evidence to support any of the proposed theories, although a recently published study suggests that patients who are asymptomatic have a much weaker immune response to the SARS-CoV-2 virus.
The study was conducted by researchers at Chongqing Medical University in Chongqing, China and involved an in-depth clinical and immunological analysis of 37 patients diagnosed as having been infected with SARS-CoV-2 and were asymptomatic and 37 symptomatic patients. Each of the patients in the asymptomatic group reported no relevant symptoms in the 14 days prior to testing, nor during the period of hospitalization after diagnosis.
The 37 asymptomatic patients were identified from a group of 178 patients diagnosed as being infected with SARS-CoV-2, 22 were female and 15 were male, and were aged between 8 and 75. The asymptomatic patients were found to shed the virus for longer than symptomatic patients, with patients with symptoms having a median duration of viral shedding of 14 days compared to 19 days for asymptomatic patients.
The asymptomatic patients had lower levels of IgG antibodies during the acute stage of infection when the virus could be identified in the respiratory tract. 8 weeks after being discharged from hospital asymptomatic patients had a 93.3% reduction in IgG and a 81.1% reduction in neutralizing antibodies, compared to 96.8% and 62.2% in symptomatic patients.
Asymptomatic patients also had lower levels of 18 pro- and anti-inflammatory cytokines which the researchers suggest is due to a much weaker immune response to SARS-CoV-2.
The researchers also found the antibody response to SARS-CoV-2 diminished rapidly after infection (within 2-3 months) in many of the asymptomatic patients. After 3 months, 40% of asymptomatic patients were seronegative compared to 12.9% of symptomatic patients. These findings suggest that contracting COVID-19 may not provide immunity for long.
You can read more about the study in the paper – Clinical and immunological assessment of asymptomatic SARS-CoV-2 infections – which was recently published in the journal Nature Medicine. DOI: 10.1038/s41591-020-0965-6