When Are People With Covid 19 Infectious?

When is Covid 19 is spread from one person to another?  A new study by researchers at the Imperial College, London, claims that two-thirds of cases were still infectious five days after their symptoms began, and one-quarter were still infectious at seven days.

The authors claimed this as the first real-world study into when people with COVID-19 are infectious. The researchers held detailed daily tests from when people were exposed to SARS-CoV-2 to look at how much infectious virus they were shedding throughout their infection.

In the study, the researchers say that in people who develop symptoms, the majority are not infectious before symptoms develop, but two-thirds of cases are still infectious five days after their symptoms begin.


“We closely monitored people in their homes from when they were first exposed to the virus, capturing the moment when they developed infection through until they ceased being infectious. Before this study we were missing half of the picture about infectiousness, because it is s hard 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 fundamental to controlling any pandemic and has not been previously defined for any respiratory infection in the community,” said study author, Professor Ajit Lalvani, Director of the NIHR Health Protection Research Unitin Respiratory Infections at Imperial, said:

“Combining our results with what we know about the dynamics of Omicron infections, we believe that the duration of infectiousness we’ve observed is broadly generalisable to current SARS-CoV-2 variants, though their infectious window may be a bit shorter. Our evidence can be used to inform infection control policies and self-isolation guidance to help reduce the transmission of SARS-CoV-2,” the author said.

Co-author, Dr Seran Hakki said: “Our study is the first to assess how long infectiousness lasts for, using real life evidence from naturally acquired infection. Our findings can thus inform guidance as to how to safely end self-isolation.”

She 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 at home and minimise contact with other people.”

The participants completed daily questionnaires about their symptoms and did daily nasal and throat swabs that were sent to a laboratory for PCR-testing. PCR-positive samples were then tested to determine if 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 lateral flow tests were at identifying actual infectiousness as opposed to PCR-positivity.

Samples from a total of 57 people were used, but not all were included in some analyses because of some participants not sharing information about their symptoms and some people shedding infectious virus before or beyond 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 started and three were asymptomatic.


The study found that the overall median amount of time that people were infectious was five days. Th researchers said that only one in five participants were infectious before symptom onset (7 out of 35 cases).

Although levels of infectiousness reduced during the course of infection, 22 of 34 cases continued to shed infectious virus five days after symptoms began, and eight of these people continued to shed infectious virus at seven days.


Most common symptoms:

Fever, cough, tiredness, loss of taste or smell

Less common symptoms:

sore throat, headache, aches and pains, diarrhea, a rash on skin, or discolouration of fingers or toes, red or irritated eyes

Serious symptoms:

difficulty breathing or shortness of breath, loss of speech or mobility, or confusion and chest pain


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