So far, the SARS-CoV-2 coronavirus has spread almost unhindered – despite all countermeasures. How long the pandemic will last is unclear. A decisive factor for this is, among other things, the question of how seasonal the virus is: Does it thrive like many influenza viruses primarily in the cold season or does it remain infectious throughout the year regardless of the climate? So far, there is little, contradictory data on this. Some studies suggest that closely related corona viruses are sensitive to heat. On the other hand, Covid-19 has also spread to tropical regions in Asia.
Many diseases are known to be rampant depending on the season. Infections transmitted by mosquitoes and other insects often peak in spring and summer. Colds and influenza, on the other hand, spread especially in the cold season. So far, most pathogens have only partially clarified why this is the case. However, temperature and humidity seem to play an important role here, as well as possibly an increased vulnerability of the human immune system in the dark season. Spending more time in closed, heated rooms in winter can also promote the spread and spread of such viruses.
But what about the currently rampant coronavirus Sars-CoV-2? The reaction of this pathogen to the approaching warmer season could be decisive for how the global pandemic situation will develop in the coming months – and how bad the consequences will be. The outbreak of the disease in China in December and the fact that so far mainly countries in the northern hemisphere have been particularly badly aroused the hope that Covid-19 could also be a seasonal infection. So far, there is little data to support this assumption. The SARS-CoV-1 virus, which was most closely related to the new corona virus and appeared in Asia in autumn 2002, was countered by early summer 2003. It is therefore unclear whether the epidemic would have subsided on its own in the warm season. The also related virus MERS-CoV only sporadically jumps from camels to humans and has not triggered a worldwide wave of infections.
Some corona viruses are seasonal
But there is some data from other Sars-CoV-2 related coronaviruses that can cause colds and minor respiratory diseases. Some of these actually seem to show a pronounced preference for the cold season, as, among others, Kate Templeton Templeton from the University of Edinburgh and her team have determined. In the analysis of the epidemiology of four such coronaviruses, they demonstrated that at least three of them have a pronounced seasonality: “In general, the coronaviruses showed a clear preference for the winter between December and April and were undetectable in the summer months,” report and her team. “This is comparable to the pattern that we see with influenza viruses.” However, this pattern was less clear for the fourth of the coronaviruses they examined. A study from the United States also found a peak in infections with two of the common cold coronaviruses in February. Like SARS-CoV-2, SARS and MERS-CoV, these belong to the so-called beta corona viruses and are therefore closely related to the new corona virus.
There are also indications that the group of enveloped viruses, which also include corona viruses, could be more sensitive to weather conditions. In a comparative study of various respiratory viruses, University of Edinburgh’s Rory Price and colleagues measured the seasonality of rhinoviruses, influenza viruses, various RSV viruses, and others using 52,060 samples taken at different times over the course of six years, examined. They also determined which weather conditions prevailed during the infections. It was found that non-enveloped viruses such as rhinoviruses and adenoviruses are present throughout the year. These viruses therefore appear to be less sensitive to seasonal fluctuations in temperature and humidity. In contrast, enveloped viruses, which include not only influenza and RSV, but also the corona viruses. Most of them showed a seasonality, as Price and his colleagues found.
So far no weakening in warm areas
As long as data on the behavior of Sars-CoV-2 are limited, it is difficult to determine whether this corona virus will behave seasonally or not. In addition: First evaluations of the spread of Covid 19 in China show that the virus has apparently spread there both in temperate regions and in parts of the country with a tropical, warm climate. “Our results suggest that changes in the weather alone will not necessarily reduce the number of Covid cases,” said Wei Luo of Harvard Medical School and colleagues. Stephen Kissler from Harvard University and his team come to a similar conclusion. They had simulated the spreading behavior of the virus on the basis of the epidemiological data to date. “In all of the scenarios we modeled, the virus generated substantial outbreaks regardless of the time of year,” they report. However, epidemic beginnings in autumn and early winter led to more acute outbreaks, while epidemics that erupted in late winter and spring caused longer-lasting outbreaks with shorter peaks.
The Kissler team also examined how the future could look in a model. This is based on the previous data on SARS-CoV-2 and the other beta-coronaviruses and simulates possible courses of the Covid pandemic until 2025. It has been shown that the future development will depend heavily on how long the one acquired through infection with the new corona virus Immunity persists. “If immunity to SARS-CoV-2 is not permanent, there will likely be regular cycles,” the researchers said. “If the protective effect lasts only around 40 weeks, as with the cold viruses HCoV-OC43 and HCoV-KU1, then annual Covid 19 outbreaks could establish themselves.” If the immunization against the virus lasts longer or even permanent, however, SARS could -CoV-2 will disappear completely after around five years and the population has been largely infected.
However: So far, all these studies and models on the possible behavior of SARS-CoV-2 have been based on only a few data, and above all only during a single outbreak. In this respect, it remains uncertain for now whether and how strongly the coronavirus may react to the approaching spring. It seems clear, however, that the pandemic will not be contained without far-reaching countermeasures – the scientists largely agree on this point.
Sources: Journal of Clinical Microbiology, doi: 10.1128 / JCM.00636-10; Scientific reports, doi: 10.1038 / s41598-018-37481-y; medRxiv, doi: 10.1101 / 2020.03.04.20031112; doi: 10.1101 / 2020.02.12.20022467