The first case of COVID-19 was reported in December 2019 in Wuhan, China. As our understanding of the disease is expanding, experts around the world have been indicating that there may be COVID-19 outbreaks in specific seasons every year like the flu.
Now, a group of researchers at the American University of Beirut in Lebanon claim that the disease may become seasonal in temperate countries but only after we achieve herd immunity against this infection.
As per a paper published in the journal Frontiers in Public Health, cold and dry temperatures during the winter season increase the stability and transmission rate of the virus and weaken host immunity, leading to increased incidence of upper respiratory tract infections.
Seasonal nature of viral respiratory tract infections
Almost every acute viral disease has a seasonal nature, which varies as per the environmental conditions and geographical location. Respiratory tract infections occur more commonly during the winter season in temperate areas and during the rainy season in tropics.
There are four types of coronaviruses (HCoV) that cause the common cold - 229E, NL63, OC43 and HKU1. These coronaviruses are responsible for about 15-30 percent of all respiratory tract infections in a year. Usually, these infections are mild but can become severe sometimes.
Previous studies have suggested that HCoV common cold cases rise during winter and spring in the temperate region, while in tropical climates, the viruses circulate throughout the year with a rise in cases in some months. For example, a study done in China indicated more HCoV common cold infections in the fall and spring months. Studies conducted in Hong Kong and Thailand found NL63 infections more common in spring, summer and autumn and OC43 in winter. In Africa, HCoV infections have been noted all through the year.
Factors affecting viral transmission
Studies on respiratory viruses have previously shown that various factors contribute to viral transmission. These include:
Environmental factors: Lower humidity and cold temperature contribute to viral stability, making the virus easily spread in a population.
This is especially true in the case of enveloped viruses like the flu virus. The envelope is a phospholipid bilayer that is present around some (but not all) viruses. The layer promotes attachment and entry of the virus into host cells. Experts suggest that low temperatures increase the ordering of lipids in this envelope, which, in turn, helps the virus stay stable for long periods of time outside the host.
Low humidity or dry climate quickly reduce the size of respiratory droplets. The smaller the droplets, the longer they can remain suspended in the air. Contrary to this, high humidity increases droplet size, reducing aerosol transmission.
However, it has been seen that flu virus can spread even in high humidity. This is because the virus remains stable in respiratory droplets once they have fallen on surfaces. This is proposed to be a possible cause of year-round infections in tropical areas.
More inactivation by UV radiation is yet another factor that may reduce the transmission of the virus in summers in temperate regions.
Host factors: It has been shown that low temperatures have an immunosuppressive effect. It also reduces the ability of the nasal mucosa to clear out pathogens. Reduction in lung function has been seen in COPD and asthma patients during colder weather. Low vitamin D levels, due to less exposure to sunlight, is suggested to have a role in immune suppression in cold weather.
As per the media reports, the authors of the study claimed that COVID-19 cannot be seasonal right now because of the presence of a susceptible host population. However, given the dynamics of transmission of the virus, which are very similar to that of other respiratory viruses including the common cold-causing HCoVs, it is possible that COVID-19 would follow suit and become a seasonal infection once herd immunity is achieved through mass vaccinations or natural infections.
However, until then, we need to keep taking necessary precautions to slow down the spread of the virus and reduce the burden on healthcare facilities.
For more information on COVID-19 and the flu, read our article on COVID-19 vs flu: How to tell the difference.
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