Study Results Show COVID-19 Becomes Infectious When Symptoms are Present


Investigators report that while lateral flow tests do not detect the start of infectiousness well, they can identify individuals who will not infect others and can safely leave isolation.

In individuals who develop symptoms of COVID-19, the majority are not infectious before symptoms develop, but two-thirds are still infectious 5 days after symptoms begin, results of a study published in The Lancet Respiratory Medicine showed.

The study, which included 57 individuals with mild COVID-19, is the first to show how long infectiousness lasts after natural infection in the community.

Investigators from the Imperial College of London in England reported that while lateral flow tests do not detect the start of infectiousness well, those tests can more accurately identify someone who is no longer infectious and can safely leave isolation.

“Before this study we were missing half of the picture about infectiousness because it’s hard to know when people are first exposed to [severe acute respiratory syndrome coronavirus 2]SARS-CoV-2 and when they first become infectious. By using special daily tests to measure infectious virus, not just [polymerase chain reaction (PCR)], and daily symptom records we were able to define the window in which people are infectious,” Ajit Lalvani, director of the NIHR Health Protection Research Unit in Respiratory Infections at Imperial, said in a statement.

“This is fundamental to controlling any pandemic and has not been previously defined for any respiratory infection in the community,” he said.

Investigators followed individuals who were exposed to someone with a PCR-confirmed COVID-19 infection in their homes between September 2020 and March 2021 and between May 2021 and October 2021. Individuals were unvaccinated and vaccinated.

Individuals completed daily questionnaires about their symptoms and also did daily nasal and throat swabs that were sent to a laboratory for PCR testing.

Investigators evaluated PCR-positive samples to determine if they contained infectious virus and how infectious the virus was. They also completed 652 lateral flow tests on the samples to determine how accurate the lateral flow tests were at identifying the infectiousness compared with PCR positivity.

Although 24 of 38 individuals tested positive via PCR tests before they developed symptoms, investigators said this does not indicated infectiousness, and most individuals were only infectious after they developed symptoms.

Only 7 of 35 of the individuals were infectious before symptom onset.

Although infectiousness decreased during the course of infection, 22 of 34 individuals continued to shed infectious virus 5 days after symptoms began, but just 8 continued to shed infectious virus at 7 days.

Investigators also found that the sensitivity of the lateral flow tests in identifying individuals who were infectious was poor at the start of infection but high after peak levels of infectiousness, at 67% and 92%, respectively.

“NHS guidance for those with symptoms but who test negative is less clear about how long people should isolate for. Our study finds that infectiousness usually begins soon after you develop COVID-19 symptoms,” Lalvani said.

“We recommend that anyone who has been exposed to the virus and has symptoms isolates for five days, then uses daily lateral flow tests to safely leave isolation when two consecutive daily tests are negative,” he said.


First real-world study gives detailed new insights into when people with COVID-19 are infectious. News release. EurekAlert. August 18, 2022. Accessed August 19, 2022.

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