Infectious respiratory diseases spread when a healthy person comes in contact with virus particles expelled by someone who is sick — usually through a cough or sneeze. The amount of particles a person is exposed to can affect how likely they are to become infected and, once infected, how severe the symptoms become.
The amount of virus necessary to make a person sick is called the infectious dose. Viruses with low infectious doses are especially contagious in populations without significant immunity.
The minimum infectious dose of SARS-CoV-2, the virus that causes Covid-19, is unknown so far, but researchers suspect it is low. “The virus is spread through very, very casual interpersonal contact,” W. David Hardy, a professor of infectious disease at Johns Hopkins University School of Medicine, told STAT.
A high infectious dose may lead to a higher viral load, which can impact the severity of Covid-19 symptoms.
Viral load is a measure of virus particles. It is the amount of virus present once a person has been infected and the virus has had time to replicate in their cells. With most viruses, higher viral loads are associated with worse outcomes.
“The more viral particles that get into the lungs, the more damage to the lungs that is probably happening,” said Hardy.
One study of Covid-19 patients in China found that those with more severe symptoms tended to have higher viral loads.
“It’s not proven, but it would make sense that higher inoculating doses will lead to higher viral loads, and higher viral loads would translate into more pathogenic clinical courses,” said Dan Barouch, director of the Center for Virology and Vaccine Research at Beth Israel Deaconess Medical Center.
People with higher viral loads may also shed more whole viruses, which makes them more contagious, compounding the danger of spreading disease more widely.
If exposure to higher doses, or even frequent low doses, of SARS-CoV-2 does lead to worse health outcomes, there are significant implications for health care workers who are routinely exposed to Covid-19 patients.
“Someone caring for large numbers of patients on the wards, if they’re not wearing PPE [personal protective equipment], there might be a high frequency of exposure as well as a high dose of exposure,” Barouch said.
In Italy, a country particularly hard-hit by the virus, about 9% of reported cases were health care workers. Here in the U.S., 10% of Covid-19 cases in California were health care workers, according to the California Department of Public Health.