Evidence suggests pandemic coronavirus may be seasonal

A new Jama Network Open cohort study found that the distribution of community outbreaks of COVID-19 is ‘consistent with the behaviour of a seasonal respiratory virus’, affected by humidity, latitude and temperature.
 
Temperature and humidity are known to affect the transmission of seasonal respiratory viruses, such as the flu.
 
But the betacoronaviruses related to SARS-CoV-2 that caused the respiratory disease severe acute respiratory syndrome (SARS) in 2003 and Middle East respiratory syndrome (MERS) in 2012 are not believed to be seasonal, though both had much more restricted transmission.
 
The new study, led by Dr Muhammad Sajadi of the Institute of Human Virology at the University of Maryland, examined data from 50 cities around the world.
 
It found that the cities with substantial community transmission of SARS-CoV-2 tended to be distributed along a latitude corridor with consistently similar weather patterns, such as mean temperatures between 5–11^OC, as well as low humidity.
 
These eight cities spanned the world, including Wuhan, Tokyo, Qom, Milan, Paris and Seattle.
 
In the early spread of the virus, the study found substantial community transmission took place on a ‘narrow band of latitudes in a consistent east and west pattern’.
 
‘Besides potentially prolonging half-life and viability of the virus, other potential mechanisms associated with cold temperature and low humidity include stabilization of the droplet, enhanced propagation in nasal mucosa, and impaired localized innate immunity, as has been demonstrated with other respiratory viruses,’ the authors wrote.
 
The new study suggests that the virus may spread better in colder weather, paving the way for estimates of likely locations of new viral hotspots.
 
Australian researchers recently found that a 1% drop in humidity can increase the number of COVID-19 cases by 6%. Low humidity makes it easier for the virus to spread, as dry air can hold aerosolised virus particles for much longer.
 
Chinese researchers recently found a similar link in analysis of data from 30 Chinese provinces, finding increases in relative humidity led to a decrease in confirmed daily cases, though the associations were not consistent across the country.
 
Other researchers suggested a seasonal link as early as February.
 
If the novel coronavirus is seasonal, that suggests Australia and other nations in the southern hemisphere may be more likely to see an uptick in viral activity during these winter months.
 
The main viral hotspots at present are in Central and South American nations such as Brazil and Mexico.
 
Brazil has almost 900,000 cases – second only to the more than two million cases in the US, according to Johns Hopkins University data.
 
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