What’s in an Incubation Period?
Over the weekend I was preparing to talk about the incubation period of COVID-19, but got sidetracked. That turned out to be a good thing, because on Monday, researchers from multiple universities released a review of 181 cases outside China about that very thing.
The incubation period for a disease is the time between when a person is exposed and when they develop symptoms. This period varies from pathogen to pathogen. Knowing this is essential in public health planning, because we need to know how long contacts to a case must be followed before they can be declared uninfected.
Knowing when you were exposed is the first challenge. Unless we had known contact with someone we knew was infected at the time (or have since learned they were infected), we may not know when we were exposed.
Based on other, related viruses and the information available during the first weeks of this pandemic (I said the P word, even if WHO is holding back), the incubation period range for SARS-CoV-2 is two to fourteen days.
Some folks think this means you don’t get sick until day 14, or that you can spread the virus for 10+ days before you get sick. Not quite.
Let’s talk about the second part of that statement first.
During the incubation period, the pathogen, in this case a virus, replicates slowly at first then exponentially. Eventually there’s so much virus it bursts out of the cells and you are contagious. Your immune system can now see it and says “ACK! Foreign Invaders! Attack! Attack!” And that’s what makes you feel sick — fever is your immune system trying to kill the virus.
While it’s possible — some say probable — that COVID-19 can be contagious in the asymptomatic stage, remember that it takes that time period for the virus to reproduce. You have low levels of virus initially, and that virus isn’t getting out until it reaches that threshold. In the case of influenza, that’s 1–2 days before you show active symptoms. In COVID-19, it’s probably the same. But in flu, while important to note, it’s also not a large part of transmission. We don’t yet know if that’s the case for COVID-19.
Back to the first part of that statement.
We know that SARS-CoV, the virus that caused SARS in 2003, has an average incubation period of five days, with a range of two to fourteen days. The incubation period for MERS, a SARS-like virus that caused a deadly outbreak in the Middle East, is five to seven days, also with a range of two to fourteen days. There are some other human coronaviruses that cause illnesses like the common cold. The incubation period for these has an average of three days, ranging from two to five days.
A range is just that, the minimum number of days and the maximum number of days possible for the pathogen to cause symptoms.
The latest report didn’t substantially change what we already believed about SARS-CoV-2. The researchers looked at 181 cases with documented exposure from 50 provinces, regions, and countries outside China. The patients included men and women, and a cross-section of the ages most affected. That means it’s a fairly representative sample. Is a sample size of 181 cases large enough to draw strong conclusions given we’ve had over 100,000 cases by now? Not quite, but it certainly gives us more and better information upon which to base our assumptions.
The researchers found that, on average, it took 5.1 days from exposure to a known case for someone to develop symptoms. By 11 days post exposure, 97.5% had developed symptoms. So that 2–14 day range is pretty accurate. The authors state that the current Centers for Disease Control guidelines are supported by the epidemiological data. In fact, >99% of patients considered “high risk” for severe disease were symptomatic well within that 14-day period.
The challenge though, was in the upper limit. It took 15 days for 4 (2.5%) of these cases to develop symptoms. CASES are people who are sick with COVID-19, not all those exposed and not the number in the population. Out of every 10,000 people who have COVID-19 (the US currently has 808 cases), 101 of those will have developed symptoms at 15 days.
Quarantine is a disease control measure that has a cost. Not just a financial cost to housing, monitoring, and feeding the people in quarantine. Removing a person’s personal liberties is not something we take lightly. It has a cost on the individual’s mental health, their physical health, their families, their financial well being, which can in turn take a toll on society.
While quarantine is sometimes necessary, we have to weigh the benefits against those costs. Some experts have reviewed this study and agree its findings are sound, but doubt that the current protocol should be changed. For some of us, it’s easy to say “just a few more days,” but it is critical that we base those decisions on sound public health principles rooted in epidemiological evaluation of each disease.