The science behind why fall weather could make COVID-19 worse

A recent Australian study confirmed suspicions that there may be a link between lower humidity and an increase in COVID-19 spread. 

The study, focused on the Greater Sydney Area, was led by professor and epidemiologist Michael Ward. This is the second peer-reviewed study looking at the relationship between weather conditions and COVID-19 in Australia. Ward said this second study confirms that humidity is a key factor in coronavirus spread

“Dry air appears to favor the spread of COVID-19, meaning time and place become important,” he said. “Accumulating evidence shows that climate is a factor in COVID-19 spread, raising the prospect of seasonal disease outbreaks.”

The research estimated that for every 1% decrease in relative humidity, there could be a 7-8 percent. Increase in COVID-19 cases

The relationship between humidity and viral infection spread is not new to COVID-19. In 2013, the Center for Disease Control and Prevention (CDC) investigated a similar link between humidity and the spread of influenza. 

That study revealed that up to 77% of infectious influenza viruses could still be infectious after one hour if the relative humidity was at 23% or less. However, when the humidity was raised to 40% or more, only 14% of infectious viruses were left in the room.

A prior study led by Dr. Jeffrey Shaman in 2010 comparing flu death rates to humidity readings showed a similar correlation. The researchers noted that there was often a significant drop in humidity prior to an influenza outbreak. 

“This dry period is not a requirement for triggering an influenza outbreak, but it was present in 55-60% of the outbreaks we analyzed, so it appears to increase the likelihood of an outbreak,” Shaman said. “The virus response is almost immediate; transmission and survival rates increase and about 10 days later, the observed influenza mortality rates follow.”

Ward explained why humidity has such a dramatic effect on the viral spread.

“When the humidity is lower, the air is drier and it makes the aerosols smaller,” he said. “When you sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer. That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.”

Ward said this is why it is so important to wear a mask — “both to prevent infectious aerosols escaping into the air in the case of an infectious individual, and exposure to infectious aerosols in the case of an uninfected individual.”

This discovery also raises concerns about heading into the winter season and a possible resurgence of coronavirus infections. Ward said it’s important for people to be careful going into a dry winter. 

“COVID-19 is likely to be a seasonal disease that recurs in periods of lower humidity. We need to be thinking if it’s wintertime, it could be COVID-19 time,” he said. 

However, immunobiologist and Yale University professor Akiko Iwasaki stressed that people can transmit the virus any time of year through contact with contaminated surfaces and people.

Ward also added that, depending on the climate of different locations, winter may not be the only season you need to watch out for.

“When it comes to climate, we found that lower humidity is the main driver here, rather than colder temperatures,” he said. “It means we may see an increased risk in winter here when we have a drop in humidity. But in the northern hemisphere, in areas with lower humidity or during periods when the humidity drops, there might be a risk even during the summer months.”

One thing to consider investing in is a humidifier, to help decrease the chance of viral spread in your home. 

Iwasaki noted that, in the winter, when cold, dry air comes in through the doors and windows and is warmed, it decreases the relative humidity in your home by about 20 percent. A humidifier can help combat this significant drop. 

But no matter what, the most important thing is to continue taking every precaution possible.  

“It doesn’t matter if you live in Singapore, India, or the Arctic, you still need to wash your hands and practice social distancing,” Iwasaki said.