University of Minnesota leads work group on infectious dose of COVID-19

As COVID-19 cases continued to ramp up across the state on Saturday, a University of Minnesota researcher reported progress from a work group that's studying how much of the pandemic virus it takes to become infected.

Scientists don't yet know the infectious dose of the virus that causes COVID-19, but data from animal studies and evidence from other viruses could provide important clues, said Michael Osterholm, director of the U's Center for Infectious Disease Research and Policy.

Osterholm convened the group to develop a consensus statement that could help evaluate the effectiveness of different measures to control the spread of the virus — ranging from indoor air ventilation systems to personal protective equipment.

"This is really an important concept because of the mistaken belief among many that it's more like a game of tag and you're it — you just come close to somebody," Osterholm said.

"Clearly an infectious dose is a function of the concentration of virus in the air that you breathe and the particle size … the virus is in — what are you inhaling," he said. "Third is the duration of that exposure in time."

The Minnesota Department of Health reported four more deaths from COVID-19 on Saturday as the state saw one of its biggest one-day increases in confirmed cases. The net increase of 804 new cases marked only the fourth time the state exceeded a daily case count of 800 new infections since the start of the pandemic.

The jump came on a much larger volume of tests than in May, when Minnesota previously broke the 800-case mark.

But the increase in cases is not solely a function of more testing and continues a troubling trajectory, said Kris Ehresmann, the state's director of infectious disease.

"We're seeing the impact of more people out socializing, visiting bars and restaurants, going to events — all of those kinds of things," she said. "We're paying very close attention to it."

The expert panel includes about 20 researchers in aerobiology, animal models, respiratory protection and infection control. Osterholm did not name the participants but said the group includes scientists from around the world.

The goal is not to launch new studies, he said, but to assemble and evaluate information being published in disparate places.

To understand the role of infectious dose, the panel is looking at the presence of virus in exhaled air, including respiratory droplets small enough to be described as aerosols. The potential importance of spread via aerosols was underscored last week, Osterholm said, when the World Health Organization (WHO) acknowledged that airborne transmission may be possible in certain circumstances.

"I still believe distancing is by far the most important aspect of protecting oneself from this virus," Osterholm said. "Aerosols ... surely have even more implications in terms of distance, just because of how far they can travel, but then how infectious are they — how much virus is there?"

The group will use mathematical models that incorporate infectious dose to illustrate how concentration, time and airflow affect infection transmission in different scenarios, according to a written summary of the work. The models will illustrate the impact on limiting transmission from variables such as distance, contact time and the number of contacts.

The work group has been meeting through online video calls and hopes to release its results "within the month," Osterholm said.

Dr. Sallie Permar, a professor of immunology at Duke University, said she thought the work group could bring forward important information, particularly as schools think about how to safely reopen. Information on the infectious dose could help prioritize whether to spend money on air filtration systems, cleaning products or other mitigation strategies.

The scientific puzzle with infectious dose has been clear, Permar said, from the shifting guidance with SARS CoV-2, which is the formal name for the virus that causes COVID-19. She noted, for example, how experts have become much more supportive of people wearing masks to stop the spread.

"I think that it is good to get a better understanding and consensus on this so that guidelines can be developed," Permar said. "There are probably some that are more effective — like masks have risen to the top — but where do air filtration systems fall? I don't think we have an answer to that."

Limited information on the infectious dose with COVID-19 is "a knowledge gap," said Dr. Mark Schleiss, a pediatrician and investigator with the Institute for Molecular Virology at the U.

Beyond having the potential to help public health efforts, the group's work could promote a better understanding of the disease itself.

"A lot of people are asymptomatic carriers, it appears," Schleiss said. "We don't know what the host and viral factors are that predict which people that become infected are going to progress to have disease, but it's probably dose-related. It is for other infections."

The question of infectious dose is important, but it's not clear how much difference it will make in terms of public health guidance, said Dr. Priya Sampathkumar, an infectious disease physician at Mayo Clinic. There are technical challenges, she added, with even answering the question.

"Unfortunately, we've had so many cases that we have a pretty good sense for how the virus behaves in people, which I think is what we need to focus on," Sampathkumar said.

But Osterholm maintains the work "would allow us to really make more sound recommendations on how individuals can protect themselves."

"Can I go to the grocery store for X minutes, and feel some confidence that if I'm in that grocery store — using some sort of respiratory protection — what might be my exposure risk?" he said. "People don't really have that kind of guidance."

On Saturday, all four of the newly reported COVID-19 deaths in Minnesota were among residents of long-term care and assisted-living facilities. Statewide, the pandemic's toll reached 1,499 deaths. The number of Minnesotans dying is down significantly, from more than 20 per day in May to about five per day last week.

Ehresmann attributed the shift to significant improvements with handling the pandemic in long-term care facilities. In recent weeks, COVID-19 cases have grown significantly in young adults who, while not invincible, run a lower risk of death or serious illness.

The concern is that "these people do not live in a vacuum," Ehresmann said. "They are working and interacting in lots of different places. So, down the road, we definitely could see additional transmission in other age groups ... because of these cases."

Saturday's latest numbers show 241 patients were hospitalized, compared with 227 on Friday. The Health Department said 121 patients required intensive care, compared with 124 on Friday. Daily tallies for hospitalized patients in Minnesota have been trending down or holding steady in recent weeks.