First real-world study provides detailed new insights into when people with COVID-19 are infectious
- Only one in five participants was infectious before COVID-19 symptoms began.
- Two-thirds of cases were still infectious five days after symptoms began, and a quarter were still infectious at seven days.
- Lateral flow tests do not reliably detect the onset of infectiousness, but can be used to safely shorten self-isolation
Researchers recommend that people with COVID-19 isolate for five days after symptoms begin and perform lateral flow testing starting on day six. If tests come back negative two days in a row, it is safe to come out of isolation. If a person continues to test positive, he or she must remain isolated while testing positive, but may disconnect on the 10th day after symptoms began. Current NHS guidance suggests people should try to stay at home and avoid contact with other people for just five days.
A new study of 57 people with mild COVID-19 estimates how long people are infectious and when they can safely come out of isolation.
The research, led by Imperial College London and published in the journal The Lancet Respiratory Medicine , is the first to reveal how long infectiousness lasts after natural COVID-19 infection in the community. The study team conducted detailed daily testing since people were exposed to SARS-CoV-2 to see how much infectious virus they were shedding during their infection.
The findings suggest that in people who develop symptoms, most are not infectious before symptoms develop, but two-thirds of cases remain infectious five days after symptoms begin.
They also suggest that while lateral flow tests do not detect the onset of infectiousness well, they more accurately identify when someone is no longer infectious and can safely leave isolation.
Study author Professor Ajit Lalvani, director of the NIHR Health Protection Research Unit in Respiratory Infections at Imperial, said: “We closely monitored people in their homes from the first time they were exposed to the virus, capturing the time when they developed the infection until they were no longer infectious. Before this study, we were missing half the picture about infectiousness, because it is difficult to know when people are first exposed to SARS-CoV-2 and when they first become infectious. By using special daily tests to measure infectious virus (not just PCR) and daily symptom records, we were able to define the window in which people are infectious. “This is critical to controlling any pandemic and has not been previously defined for any respiratory infection in the community.”
“Combining our results with what we know about the dynamics of Omicron infections, we believe that the duration of infectiousness we have observed is broadly generalizable to current SARS-CoV-2 variants, although their infectious window may be a little longer. short. “Our evidence can be used to inform infection control policies and self-isolation guidance to help reduce the transmission of SARS-CoV-2.”
Co-author Dr Seran Hakki, also from Imperial’s National Heart and Lung Institute, said: "There is no longer a legal requirement to self-isolate if you test positive for COVID-19, but most people still want isolate themselves until they are no longer infectious.
Despite this, there is a lack of clarity on how to exit self-isolation safely. Our study is the first to assess how long infectiousness lasts, using real-life evidence of naturally acquired infection. “Our findings can therefore inform guidance on how to safely end self-isolation.” It adds: “If you test positive for COVID-19 or have symptoms after being in contact with someone with confirmed COVID-19, you should try to stay home and minimize contact with other people.”
The most complete picture of the course of infectiousness to date
Previous studies that estimated how long someone is infectious have been a lab-based human challenge study or have used mathematical models. The new study followed people who were exposed to someone with PCR-confirmed COVID-19 in their home between September 2020 and March 2021 (pre-Alpha SARS-CoV-2 virus and Alpha variant waves) and May-October 2021 (Delta variant wave). ), including some who were vaccinated and others who were not.
Participants completed daily questionnaires about their symptoms and took daily nasal and pharyngeal swabs that were sent to a laboratory for PCR testing. PCR-positive samples were then tested to determine whether they contained infectious virus and how infectious the virus was. The researchers also completed 652 lateral flow tests on the samples to determine how accurate the lateral flow tests were at identifying true infectivity compared to PCR positivity.
Samples from a total of 57 people were used, but not all were included in some analyzes because some participants did not share information about their symptoms, some people did not shed culturable viruses, and some people shed infectious viruses before or after the sampling period. As a result, the duration of infectiousness was measured in 42 people. There were 38 people with a confirmed date of when their symptoms began and three were asymptomatic.
Real-world timeline of infectiousness
The study found that the overall average amount of time people were infectious was five days.
Although 24 of 38 people tested positive in a PCR test before developing COVID-19 symptoms, this does not indicate infectiousness and most people only became infectious after developing symptoms. Only one in five participants was infectious before the onset of symptoms (7 of 35 cases).
Although infectious levels decreased over the course of infection, 22 of 34 cases continued to shed infectious virus five days after symptoms began, and eight of these individuals continued to shed infectious virus at seven days. Current NHS guidance suggests people should try to stay at home and avoid contact with other people for just five days.
Safe self-isolation
To help understand when people can safely come out of isolation, researchers compared infectiousness levels with lateral flow test results.
They found that the sensitivity of these tests in identifying when someone was infectious was low at the beginning of infection, but high after peak levels of infectiousness (sensitivity of 67% vs. 92%, respectively). This suggests that lateral flow tests are good at detecting when someone is no longer infectious and tests to release people from isolation may work, but are not reliable for early diagnosis unless used daily.
Based on their findings, the researchers recommend that people with COVID-19 isolate themselves for five days after symptoms begin and then complete lateral flow testing starting on day six. If these tests are negative two days in a row, it is safe to leave isolation. If a person continues to test positive or does not have access to lateral flow devices, they should remain isolated and, to minimize transmission to others, should only leave on the 10th day after symptoms began.
Professor Lalvani said: “Self-isolation is not required by law, but people who want to isolate need clear guidance on what to do. The NHS currently recommends that if you test positive for COVID-19 you try to stay home and avoid contact with other people for five days , but our data suggests that, in a crude five-day self-isolation period, two-thirds of "cases released into the community would still be infectious , although their level of infectiousness would have been substantially reduced compared to earlier in the course of their infection."
“NHS guidance for those with symptoms but testing negative is less clear about how long people should isolate. Our study finds that infectiousness typically begins shortly after developing COVID-19 symptoms. “We recommend that anyone who has been exposed to the virus and has symptoms isolate for five days, then use daily lateral flow testing to safely exit isolation when two consecutive daily tests are negative.”
The study did not evaluate currently circulating Omicron variants. There is some evidence that Omicron variants have a lower viral load and are shed for less time than other variants, and the researchers note that their recommendations may be cautious, but still applicable, if this is true.
Most study participants were white, middle-aged, had a healthy BMI, and had no medical conditions. In other age groups and those with medical conditions, these results may vary as they may be slower to clear the virus.
A person’s infectiousness is one factor involved in transmission, along with environmental and behavioral factors, such as where people mix and whether they are in close proximity to each other.
Implications of all available evidence Our results uniquely define the window and kinetics of SARS-CoV-2 infectivity in naturally acquired infection. Furthermore, our findings suggest that recent observations in the controlled experimental human challenge model are largely generalizable to community cases of COVID-19. However, there was broader interindividual variability in the duration and amount of infectious viral shedding in our larger real-world cohort. This likely reflects the greater demographic heterogeneity of community cases as well as variability in dose and route of infection for community transmission events compared to highly controlled experimental inoculation of preselected healthy volunteers in the challenge. By outlining the period of infectiousness in mild COVID-19 and its correlation with symptom onset and commonly used diagnostic tests, our findings allow isolation guidance to be calibrated with respect to the infectious window . |
The study was funded by the National Institute for Health and Care Research.
