April 22, 2020, 11:45 am | Read time: 5 minutes
People infected with the coronavirus can transmit it to others even before showing symptoms or after officially recovering–most of us have heard this in some form. But when exactly are you contagious? Research is ongoing to define this critical period of infectiousness more precisely. Some results are already available.
The more patient case data available to researchers, the better they understand the SARS-CoV-2 virus and the COVID-19 disease. A key focus is determining how contagious infected individuals are and when they pose a risk to others.
Study on Virus Multiplication at the Moment of Infection
Charité’s chief virologist, Prof. Dr. Christian Drosten, is aware of the research findings; some were obtained under his leadership. In a late March episode of the “Corona Update” podcast on NDR, he discussed a recent study from Hong Kong. Ninety-six cases were analyzed, showing the same observations as his team’s smaller study weeks earlier: the virus multiplies immediately after infection.
The coronavirus SARS-CoV-2 is “already highly detectable in the earliest swabs,” Drosten reports, indicating that within the first two days, it is “on the decline in swabs. This means the virus peak must occur before the first day.” With each subsequent swab, the virus presence decreases.
But What Does This Mean for Infectiousness?
In the same context, the research group examined when infected individuals become contagious. Together with scientists from Guangdong, they studied cases of pairs where one person infected the other.
The focus was on the time between symptom onset in one person and symptom onset in the other. This period, known as the serial interval, was determined relatively precisely: with an average of 5.2 to 5.8 days.
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Corona-Infected Individuals Are Contagious Almost Immediately
Prof. Drosten compared these new figures with those from a previous “very well-conducted study” by the same university. This study focused on the incubation period, the time between infection and the first appearance of symptoms or disease onset. It defined an average incubation period of 5.2 days.
“This is certainly interesting,” the expert says in the NDR podcast, “because we have a phenomenon where the serial interval is almost as long as the incubation period.”
Consequently, a patient waits as long for symptoms after infection as the person they infected. They could have transmitted their infection almost immediately afterward.
This Is the Case for Almost Half of the Infected
As Drosten explains, the probability distribution of the available data allows for calculating a peak in infectiousness. “And the so-called Area under the curve, the area covered by this probability curve before symptom onset, is 44 percent. This means that 44 percent of all infection events occurred before the infected person was even sick.”
Given this, the current distancing rules (keeping at least 1.5 meters away from people not in the same household) make even more sense. Most would naturally avoid visibly sick individuals. However, they are apparently infectious well before–or even more infectious.
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When Are You Most Contagious and for How Long?
“The peak of infectious activity, the most contagious day in the course of the illness, is the day before symptom onset on average for all patients. This is something to note,” Prof. Drosten clarifies in the NDR podcast on April 20, 2020, as he revisits the Hong Kong study (now published in “Nature Medicine”). “Patients are most infectious the day before symptoms begin. Then, most infectious activity is over after four days of symptoms. If someone has had symptoms for four days, they are almost no longer infectious. And after a week of symptoms, they are not infectious anymore.”
A study from January 2021, published on the British government’s website, reached a different conclusion nine months later. In the research, “Public Health England” regularly tested tens of thousands of healthcare workers for COVID-19 infections and antibodies. It found that most people who had recovered from an infection were immune for at least five months. However, the study found evidence that recovered patients with antibodies could still carry a high viral load and infect others. Therefore, even temporarily immune individuals should continue to follow all precautions, such as wearing a mask, maintaining a distance of at least 1.5 meters, and washing hands regularly.
All Current Findings Are Subject to Change
Virology professor Drosten warns that scientific findings are currently reaching the public particularly quickly. Normally, this step would be preceded by review processes that can take weeks to months. But time and likely capacity are lacking at the moment.
Currently, it sometimes goes “from a scientist to the journal,” without detours. It is therefore not unusual that after a while–when updated findings are obtained–corresponding adjustments are necessary. It may also be that the timing of when infected individuals become contagious will be further specified.