After nearly two years of more and more bad news, finally a potential ray of light: COVID-19 might be a seasonal illness.
Ever since the novel coronavirus first coughed its way onto the scene back in late 2019, scientists and policymakers alike have debated whether it would be seasonal – increasing in transmission during the winter, like the flu – or equally transmissible throughout the year. This week, a paper published in the journal Nature Computational Science has provided strong evidence that COVID-19 is in fact a seasonal low-temperature infection – and that has important implications for how we can learn to live with the virus.
"Altogether, our findings support the view of COVID-19 as a true seasonal low-temperature infection, similar to influenza and to the more benign circulating coronaviruses," explained Xavier Rodó, director of the Climate and Health program at ISGlobal and coordinator of the study. "This … warrants an emphasis on 'air hygiene' through improved indoor ventilation as aerosols are capable to persist suspended for longer times."
In low-humidity conditions, the paper explains, aerosol droplets can slightly evaporate in the air, making them shrink in size. That can make diseases – potentially including COVID-19 – more transmissible, as the water droplets in our breath can become small enough to get past the immune defenses in our nose. That’s why the researchers say more importance needs to be placed on good ventilation, as well as likely meteorological scenarios, in public policy and medical interventions.
This isn’t the first study to conclude that COVID-19 is likely to be seasonal – one study, published in the journal GeoHealth back in May, studied the link between environmental conditions and COVID-19 transmission rates across five countries and concluded that the virus follows a similar seasonal pattern to that of the flu. This week’s study confirms that on a much larger scale: the team looked at the spread of COVID-19 in over 160 countries across five continents, first before policy interventions were put into place, then throughout the first, second, and third waves.
Without the input of public health policies, the team found a negative relationship between the transmission rate of COVID-19 and the temperature and humidity levels across the globe – in other words, the lower the temperature or humidity, the higher the transmission rate. But as the pandemic evolved, the pattern continued: the first wave died down as temperatures and humidity levels increased, and then, as summer turned into fall, along came the second wave. This pattern held true at a worldwide level, country level, regional and even city level, with just one exception: the summer of 2020.
"[That] could be explained by several factors, including mass gatherings of young people, tourism, and air conditioning, among others," said first author Alejandro Fontal. And in fact, air conditioning might have a lot to answer for, as the study explains that “In rich countries of the Northern Hemisphere, the cooled indoor microclimate during [warmer] months could accommodate transmission, as in the many outbreaks observed in the meat-processing plants of the United States and Europe where cooled air is mechanically reventilated.”
While the team believe the results are convincing, they do note that the study had several limitations. For one thing, COVID-19 is a new virus, and therefore epidemiological records are somewhat limited. They also caution that their results show a correlation, not a causation, so while it’s likely the ebb and flow of the virus was linked to the temperature and humidity, it can’t be discounted that there might be some hitherto unknown factor at play.
Nevertheless, the team hope that their study might help inform healthcare policies and treatments going forward, allowing for “more curated and tailored climate services and early-warning systems for COVID-19.” Measures such as lockdowns, they say, can be considered in terms of limiting climate effects on the disease, and, crucially, the team hints that their methods would hold up even if the disease “were … to become endemic, therefore defining the annual timing for vaccination.”
"The question of whether COVID-19 is a genuine seasonal disease [is] increasingly central, with implications for determining effective intervention measures," Rodó said. "Altogether, our findings support the view of COVID-19 as a true seasonal low-temperature infection, similar to influenza and to the more benign circulating coronaviruses."