Thank you Sacramento Rotary Club for inviting me to speak and answer your questions over Zoom yesterday. I received some excellent questions in a follow up email so I’m sharing my answers with everyone. The quotes come from a summary review of COVID that just came out in the Journal of the American Medical Association, which offers a consensus view of the science as we currently understand it (July 2020). Here’s a link to the paper.

When people talk of a two-week quarantine, is that enough to know whether you have it or not?

“The average time from exposure to symptom onset is 5 days, and 97.5% of people who develop symptoms do so within 11.5 days.”

This means a two-week quarantine will catch nearly every single case.

How long can a person be asymptomatic and able to spread the virus?

First, this is a crucial realization, that SARS-CoV-2 can be spread by people who do not yet know they are infected. In fact, most cases probably happen this way (“An estimated 48% to 62% of transmission may occur via presymptomatic carriers.”)

To clarify, there is some confusion about terms: Presymptomatic means people who are infected, feel fine, but will eventually develop some symptoms later. Asymptomatic means people who are infected, feel fine, and are destined to never show any sign of infection.

“True asymptomatic infection is probably uncommon.”

So how long can a person be unknowingly contagious?

“…presymptomatic transmission has been described in clusters of patients with close contact (eg, through churchgoing or singing class) approximately 1 to 3 days before the source patient developed symptoms.”

Based on the quotes above, most presymptomatic spread will occur within 5 days of exposure to the virus. Conceivably a person could silently incubate the virus for as long as two weeks after exposure but it’s unlikely they would be contagious that whole time. Virus-infected individuals are most likely to infect another person the day of, and the day or two BEFORE they start having any symptoms, partly because they may not be taking proper precautions. They continue to be contagious after symptoms begin.

“Pharyngeal shedding is high during the first week of infection at a time in which symptoms are still mild, which might explain the efficient transmission of SARS-CoV-2, because infected individuals can be infectious before they realize they are ill.”

Are there people walking around for months without symptoms who may be spreading the virus?

Probably not, I haven’t seen any evidence or any reports of anything like that. Though there are definitely people who continue to have medical problems related to their infection for months after. How long the virus lingers in the body is unknown. We know that viral RNA can be detected for many weeks, but that may be leftover trash, not living virus.

Amy Rogers, MD, PhD, is a Harvard-educated scientist, novelist, journalist, and educator. Learn more about Amy’s science thriller novels, or download a free ebook on the scientific backstory of SARS-CoV-2 and emerging infections, at AmyRogers.com.

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