Scientists, policymakers and health professionals are eager to determine the extent to which COVID-19 can be seasonal. Understanding this aspect of the disease can guide our response to the pandemic.
Researchers at UC Santa Barbara have found evidence that COVID-19 distribution is sensitive to UV exposure. Although this suggests that COVID-19 may differ with the seasons, there are other seasonal factors – such as temperature, specific humidity and precipitation – whose consequences are uncertain, given the available data. The results appear in the Proceedings of the National Academy of Sciences.
“There were apparently related species of coronavirus – such as SARS from 2003 and MERS from 2012 – poorly related to temperature and humidity, but were sensitive to UV radiation,” said co-author Kyle Meng, an environmental economist at UC Santa Barbara’s economics department. and at the Bren School of Environmental Science & Management on campus.
Mix, along with Tamma Carleton, also at Bren School, Peter Huybers and Jonathan Proctor of Harvard, and Jules Cornetet at the French École Normale Supérieure Paris-Saclay, tried to explore the link between UV radiation and the spread of COVID-19 investigate by constructing a high resolution global dataset of daily COVID-19 cases.
Many articles compare different places to get an idea of how the disease responds to different environmental conditions. But this approach introduces a number of potentially confusing factors into statistical models, such as health care quality, income, and cultural norms.
Mix offers the more temperate United States and more tropical Brazil as examples. “The US and Brazil differ for a variety of reasons,” he said. “They have different economic conditions and institutions, in addition to different environmental conditions.” These distinctions, according to him, prevent a clean comparison of the distribution of COVID-19, based only on environmental conditions.
To circumvent this issue, the team conducted a longitudinal investigation, which compared many populations over time. Instead of comparing Brazil to the US, they compare communities in Brazil to themselves at a different time, when local environmental conditions changed. “We basically ask whether daily fluctuations in environmental conditions that a population experiences affect new COVID-19 cases until two weeks later,” Meng explained.
To achieve this, the researchers needed a lot of data. Unfortunately, when the team started their work, international COVID-19 datasets, such as those of John’s Hopkins, provided only data at the national level. Records with finer resolution were distributed across different agencies and institutions in different languages and formats.
“We have taken very diverse datasets from statistical agencies from different countries and harmonized them to create a global dataset consisting of more than 3,000 spatial units,” Carleton said. The authors then use a set of meteorological conditions with a daily solution to adapt local environmental conditions to the daily COVID-19 number of cases.
The researchers applied a series of statistical techniques to analyze how four variables – UV radiation, temperature, humidity and precipitation – correlate with the daily growth rate of COVID-19 cases, a measure of how quickly the disease spreads in a region. . They also estimate the delay time between changes in environmental conditions and possible effects on recorded COVID cases, which can be significant given the virus’ four- to seven-day incubation period, along with additional delays due to the test.
The team found evidence that exposure to UV at a site significantly affects COVID-19 transmission. A change in UV exposure by 1 standard deviation (approximately equal to the difference in UV between May and June in Los Angeles) reduced the growth rate of new cases over the next two weeks by approximately 1 percentage point. This could lower COVID-19 growth rates from an average daily increase at the onset of the epidemic of 13% to an increase of 12% per day.
Based on the expected seasonal changes in UV radiation, the model predicted that growth rates would rise by 7.3 percentage points between January and June for southern temperate places. Meanwhile, northern temperate regions will see a UV-driven decline of 7.4 percentage points during the same period, as longer days increase UV exposure.
This pattern changes as the seasons turn. By December, researchers predicted that COVID-19 growth rates could decline by 7.7 percentage points in southern temperate regions compared to July numbers, while cooler northern areas could see a jump of 7.8 percentage points during this period.
What is important is that the seasonal impact of UFS on the transmission of the disease is small compared to the policy of social distance, such as travel ban, school closure or isolation from home, the authors said. Regardless of the weather, social distance measures seem necessary to significantly slow down the spread.
Consistent with these findings, it appears that infection rates have decreased in the northern hemisphere during the summer, possibly due to increased exposure to UV. Meng a large portion of the northern hemisphere relaxed their COVID home visits at the same time, Meng said. As a result, there is an amalgamation between UV effects and the weakening restrictions during the summer months. “This is a major reason why our study uses daily fluctuations in UV exposure, in part to avoid confluent influences when looking at long-term, fluctuating fluctuations.”
These findings are consistent with concerns about the rise of COVID-19 infections currently being experienced in the U.S. with the advent of winter; However, to get the complete seasonal picture, researchers will need more accurate estimates of how the disease responds to other seasonally varying environmental conditions, such as temperature and specific humidity. “We are confident with the UV effect, but this is only one of the complete seasonal pictures,” Carleton said.
Laboratory studies will ultimately be critical in determining the mechanisms at work, although the authors suspect that several factors may be behind the effect of UV on COVID-19 transmission, some of which cannot be studied in the laboratory. The first is biological. UV can damage the nucleic acids that the virus uses to encode its genetic information. Co-author Jonathan Proctor, a postdoctoral fellow at Harvard, believes that UV radiation can deactivate the virus while it is being transmitted, such as when it hangs in the air or rests on an exposed surface. ‘Just as UFS can destroy our own DNA if we do not use sunscreen, UV can damage the COVID-19 virus, ”said Proctor.
Another component is behavior. For example, people can go out more often when it is sunny, which can change the risk of transmission. Although laboratory studies can help determine the biological mechanisms, population-level studies such as these can also capture social factors.
“In the context of all this, our study suggests that seasonal changes in UV may affect COVID-19 transmission in the coming months,” Meng said. “And if that’s true, we need to think carefully about how to modulate COVID-19 restriction policies in a seasonal way.”
Reference: “Global Evidence for Reducing COVID-19 Growth Rates for Ultraviolet Radiation” by Tamma Carleton, Jules Cornetet, Peter Huybers, Kyle C. Meng, and Jonathan Proctor, December 15, 2020, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / pnas.2012370118