UC Davis infectious disease experts have some warnings about COVID-19 and summer


Summer is coming and COVID-19 restrictions are easing. That means people are changing what they do both indoors and outdoors, raising new questions about staying safe.

Two UC Davis infectious disease experts explained what is changing. Both had the same strong warning.

“Stay the heck away from people,” said Jonathan Eisen, a professor at the UC Davis Genome Center and in the School of Medicine department of microbiology and immunology. “If they are not your family or cohabitants, the risk hasn’t gotten any less.”

“The most important thing people can do is maintain physical distancing,” said Dean Blumberg, UC Davis chief of pediatric infectious disease. “Summer doesn’t change that.”

Both said their second most important advice is to continue wearing a mask to reduce the spread of the virus. “If you care about protecting your friends and family and your community, you’ll wear a mask,” Blumberg said. “If there are benefits of summer and heat slowing transmissions, wearing a mask will greatly increase those.”

Generally, coronaviruses like the one causing COVID-19 tend to wane in summer for a handful of reasons. But not always, Blumberg said, and too little is known about this one to make any prediction.

“I can’t say with confidence either way, whether transmissions will continue at a high rate or whether we’ll have a break,” he said. “The H1N1 influenza in 2009, for instance, didn’t slow down at all. Right now, we still have more than 95% of the population susceptible to COVID-19 infection. It still has the potential to spread through the population like a wildfire.”

When coronavirus spread is slowed in summer, there are three factors.

  • People spend more time outdoors and often are more spread out, so they’re less likely transmit the virus.
  • Coronaviruses tend not to survive well in heat.
  • The virus also tends to have a shorter survival in humidity.

“But we just don’t know if these summer factors will have an impact,” Blumberg said. “The reason we are seeing fewer infections right now is because of the strict social distancing we’re doing.”

In fact, he said, the large beach, river and park gatherings recently could be creating another wave of infections.

“We may not know for one-to-two incubation cycles (of up to 14 days each), meaning we could have a new wave in about a month,” Blumberg said. “I can’t urge people strongly enough to avoid any large gathering where they can’t social distance, and to wear a mask when you’re out.”

One thing experts do know is that being indoors creates a higher risk of transmissions because people are closer together. And the hottest summer days tend to keep people inside.

“In Sacramento and Davis, summer also means people spend more time indoors,” Eisen said. “It’s even more important to practice social distancing and to wear a mask in indoor public places.”

Air conditioning can be a useful filter

Eisen said air conditioning, especially in homes and buildings with good filtration systems, will potentially help reduce the amount of the virus in the air. Bringing in outdoor air by opening vents or windows can also help reduce transmission risk in indoor spaces.

He said there were initial concerns that the virus was small enough to pass through air conditioning filters. Now scientists know the virus in the air is almost always attached to something such as droplets, cells or tiny bits of dirt.

“The size of the virus turns out to be not as important as the size of the particle it’s on,” Eisen said. “The well-maintained air systems of many buildings should filter out most of it.”

The experts had some additional advice about the safety of places we are likely to go and things we’re likely to do in summer.

Outdoor exercise

“A short exposure outdoors should not be a problem,” Blumberg said. “It doesn’t worry me at all on the bike trail when someone passes another person. There’s very low risk.”

Outdoor parties

“Any mass gathering is risky, but being outside helps because the air flow dilutes the virus,” Blumberg said. “That’s why it’s so important to social distance. It will provide a great deal of protection outside.”

Backyard barbecues

If people physically distance, wear a mask (when not eating or drinking), and wash or sanitize their hands, Eisen said, the transmission risk is low. But he had a few caveats:

He said there is no evidence that the virus is transmitted by food, but it can survive on surfaces and objects such as furniture, door handles and food utensils. Although the risk of transmission from these surfaces is currently unknown (his lab is studying that question), he said people can reduce the risk by avoiding those surfaces when possible and cleaning them often when they can’t, using their own utensils, and washing their hands after contact.

“The biggest issue for gatherings is likely going to be the bathroom,” Eisen said. “Even if people are not wearing a mask outside, when they come into the house to use the bathroom, that’s a good time to put one on. This would reduce risk of transmission both to and from that person.”

He suggested minimizing time indoors for bathroom visits, avoiding contact with surfaces such as door handles as much as possible, and thorough hand washing for at least 20 seconds. In addition, he said, try to space out the time between bathroom visits among guests.

The mall

“There will be a lot of people in that large building, so take social distancing very seriously and wear a mask,” Eisen said. “If it’s busy, you might want to turn around and come back another day.”

He said the biggest risk will likely be in the mall bathrooms.

“They’re small, there could be lots of people in there, and the air circulation isn’t the best,” Eisen said. “Since we don’t yet know how infectious the virus is on surfaces, if you can avoid using the restroom, you should reduce your risk of picking up the virus. I know that can be hard, but you might want to make that part of your decision process.”

Swimming pools

“The virus is not transmitted by water,” Blumberg said. “But be sure the pool is well maintained.”

For public pools, Blumberg cautioned again about avoiding crowds outside the water or in a snack bar line. “If you’re going to stay at the pool, social distance and maybe bring your own food,” he said.

Sports events

“First there is the crowd factor, even at a softball game,” Blumberg said. “And any activity that increases the virus in the air, such as singing or cheering for a team, greatly increases the risk.”