E-Paper
Most coronaviruses seasonal; Sars-CoV-2 presents mystery
Coronaviruses have long been recognised as human respiratory pathogens manifesting as mild respiratory illnesses. But when animal coronaviruses spill over to humans, they are known to cause severe diseases.
Out of the seven coronaviruses known to infect humans, four are seasonal and cause common respiratory infections that transmit similar to influenza, according to a new paper by public health researchers at the University of Michigan.
The authors of the study, published in the Journal of Infectious Diseases on April 4, say that their findings may help investigators better prepare for the Covid-19 pandemic. But it is not possible to say that SARS-CoV-2, which causes Covid-19, will also behave as the four do, they say.
The researchers used data from an ongoing 10-year household influenza vaccine evaluation study based on hundreds of individuals infected by the four mild coronaviruses: OC43, 229E, HKU1, and NL63. The rest of the three are SARS-CoV, MERS or EMC, and SARS-CoV-2.
The researchers from the epidemiology department of the University of Michigan School of Public Health found that 9% of adult cases and 20% of cases in children associated with these four coronaviruses involved doctor visits. During year-round surveillance, most coronavirus cases were detected between December and May with a peak in January-February. Only 2.5% of cases were seen between June and September.
The highest infection frequency was among children under five. Of 993 infections, about 260 were acquired from an infected household member. The transmission window for the infection acquired from a household member ranged from 3.2 to 3.6 days. Cases in children under five and adults over 50 were more likely to manifest themselves as severe disease.
Coronaviruses have long been recognised as human respiratory pathogens manifesting as mild respiratory illnesses. But when animal coronaviruses spill over to humans, they are known to cause severe diseases as in the case of the severe acute respiratory syndrome (SARS) in 2002 and Middle East Respiratory Syndrome (MERS) in 2012.
“Even though the seasonal coronaviruses found in Michigan are related to SARS-CoV-2, we do not know whether that virus will behave like the seasonal coronaviruses,” said Arnold Monto, a professor at the University of Michigan and co-author of the study, in a statement.
Amit Singh, an associate professor at Bengaluru’s Indian Institute of Science, said based on the clues from other coronaviruses that infect humans, it is possible that Covid-19 will eventually become seasonal. “A 10-year-old study conducted in the UK on three coronaviruses isolated from humans showed marked winter seasonality. Several unpublished observations seem to suggest that higher temperatures are linked to lower incidences of Covid-19. However, without data assembled over a number of seasons, the relation between Covid-19 and seasonality is mostly a prediction.”
Singh said if the incidence of Covid-19 reduces over the coming months, it could be due to a number of reasons: effective lockdown, the build-up of immunity, or high temperatures. “Since the virus is new, we do not have immunity against it. However, once infection happens and people recover, we are likely to grow immunity against the pathogen,” he said. He added most of the pathogens like to persist in the host to maintain the reservoir. “If Covid-19 mutates at a much slower rate, then the ongoing wave of Covid-19 will generate immunity in the population, which eventually reduces the transmissibility and pathogenicity of the virus.”
Dr T Jacob John, a former head of virology at Vellore’s Christian Medical College, said the future is very predictable. “At the end of 21 days, the government may extend the lockdown [imposed to check the spread of the disease]... lifting the lockdown would mean opening the floodgates. The initial spread of the infection will be very slow; a few cases here and there. Then cases will increase...like an avalanche. Maybe the speed of transmission will slow down in summer and then catch up again when the temperature falls. For prevention, people should continue to exercise physical distancing of about six feet but continue with social life and relationships.”
