Decreasing humidity in the winter is likely to lead to more COVID-19 cases

Decreasing humidity in the winter is likely to lead to more COVID-19 cases

By Angela Betsaida B. Laguipo, BSNNov 8 2020

The coronavirus disease 2019 (COVID-19) pandemic continues to ravage across the globe a month before the winter season kickstarts. Health experts fear that the cold climate may cause skyrocketing cases of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) – the causative pathogen of COVID-19.

A team of researchers from the Center for Disease Dynamics, Economics & Policy, USA, and Johns Hopkins University, USA, found evidence to support the notion that increased humidity may have contributed to lesser cases in spring earlier this year (2020). However, as the winter season fast approaches, the researchers fear that the decreasing humidity is likely to lead to an increase in COVID-19 cases.

The study

So far, the coronavirus pandemic has infected over 50 million people worldwide and claimed the lives of at least 1.25 million. In the United States, there are over 9.95 million cases and at least 237,000 deaths.

The primary mode of transmission of SARS-CoV-2 is through respiratory droplets from infected people. Recent evidence has suggested that the virus can also spread through airborne particles.

The first wave of cases in the United States came in the late winter of 2019-2020, but overall most of the cases occurred during the spring and summer. As the fall season begins in the Northern Hemisphere, the weather will become drier and colder.

Past studies have shown that falling humidity is tied to increased transmission rates of other respiratory diseases, such as influenza. Experts believe that the same is true in COVID-19, and the nearing winter may lead to a surge of new cases as the present spike in infections across the Northern Hemisphere has already been suggested.

The current study, which appeared on the pre-print server medRxiv* in November 2020, highlights the role of humidity and climate in the spread of SARS-CoV-2. To arrive at the study findings, they used dynamic time warping to cluster all 3,137 counties in the United States based on temporal data on absolute humidity between March and September.

From there, they utilized a multivariate generalized additive model that combined data on human mobility obtained from mobile phone data with humidity data. This way, they could determine the possible effect of absolute humidity and mobility on new daily cases of SARS-CoV-2 infection.


(A) Map of US Counties and their respective absolute humidity clusters. The clustering analysis was conducted using a partitional algorithm that utilized dynamic time warping (DTW) to measure similarity between absolute humidity profiles of 3,137 counties in the United States. Expectantly, the clustering of absolute humidity is related to the geography of the counties which serves as a proxy for regional weather patterns and different climatological regimes. (B) The cross-sectional smoothed mean of human encounter absolute humidity, and new case per 10,000 people trends for each cluster group of the 987 counties analyzed in the regression analysis.

Study findings

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The team found ten groups of counties with similar humidity levels in the United States. They also found a significant negative effect between increased humidity levels and new cases of COVID-19 in most regions, especially between March and July.

The effect was more significant in regions with lower humidity, such as the Midwest, Western and Northeast regions of the United States.

In the two regions that incurred the largest effect, a 1 g/m3 increase in absolute humidity has resulted in a 0.21 and 0.15 decrease in cases.

The increasing humidity seems to have played a role in the decreasing cases in the spring. They conclude, then, that increasing humidity is linked to an increase in COVID-19 cases.

“Furthermore, the fact that mobility data were positively correlated suggests that efforts to counteract the rise in cases due to falling humidity can be effective in limiting the burden of the pandemic,” the team added.

Approaching winter

The looming winter in the United States and other countries raises concerns about the COVID-19 pandemic’s prognosis over the coming months. Other respiratory illnesses, such as those caused by influenza, middle east respiratory syndrome coronavirus (MERS-CoV), and the severe acute respiratory syndrome coronavirus (SARS-CoV-1), increase in number during the cold season.

The researchers said that as the absolute humidity decreases during the winter season, COVID-19 cases may increase over the next couple of months. If this happens, health care systems across the globe will experience a further strain, which may lead to the lack of hospital beds for the severely ill patients infected with the virus.

“Increasing COVID-19 cases will pose an issue for many healthcare systems, which in normal years are typically stretched thin from regular seasonal infections, such as influenza,” the team wrote in the paper.

“Furthermore, seasonal changes in human behavior may also impact the number of new cases and hospitalizations since people are more likely to occupy indoor spaces for longer durations when outdoor temperatures decrease, thus increasing the risk of transmission,” they added.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information

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