It would be unethical to experimentally manipulate viral dose in humans for a pathogen as serious as the coronavirus, but there is evidence that dose also matters for the human coronavirus. During the 2003 SARS coronavirus outbreak in Hong Kong, for instance, one patient infected many others living in the same complex of apartment buildings, resulting in 19 dead. The spread of infection is thought to have been caused by airborne viral particles that were blown throughout the complex from the initial patient’s apartment unit. As a result of greater viral exposure, neighbors who lived in the same building were not only more frequently infected but also more likely to die. By contrast, more distant neighbors, even when infected, suffered less.

Low-dose infections can even engender immunity, protecting against high-dose exposures in the future. Before the invention of vaccines, doctors often intentionally infected healthy individuals with fluid from smallpox pustules. The resulting low-dose infections were unpleasant but generally survivable, and they prevented worse incidents of disease when those individuals were later exposed to smallpox in uncontrolled amounts.

Despite the evidence for the importance of viral dose, many of the epidemiological models being used to inform policy during this pandemic ignore it. This is a mistake.

People should take particular care against high-dose exposures, which are most likely to occur in close in-person interactions — such as coffee meetings, crowded bars and quiet time in a room with Grandma — and from touching our faces after getting substantial amounts of virus on our hands. In-person interactions are more dangerous in enclosed spaces and at short distances, with dose escalating with exposure time. For transient interactions that violate the rule of maintaining six feet between you and others, such as paying a cashier at the grocery store, keep them brief — aim for “within six feet, only six seconds.”

Because dose matters, medical personnel face an extreme risk, since they deal with the sickest, highest-viral-load patients. We must prioritize protective gear for them.

For everyone else, the importance of social distancing, mask-wearing and good hygiene is only greater, since these practices not only decrease infectious spread but also tend to decrease dose and thus the lethalness of infections that do occur. While preventing viral spread is a societal good, avoiding high-dose infections is a personal imperative, even for young healthy people.

At the same time, we need to avoid a panicked overreaction to low-dose exposures. Clothing and food packaging that have been exposed to someone with the virus seem to present a low risk. Healthy people who are together in the grocery store or workplace experience a tolerable risk — so long as they take precautions like wearing surgical masks and spacing themselves out.