COVID-19: Average Time of Infection and Other Q&A

Author: Imperial College London
Published: 2022/08/19 - Updated: 2023/10/08
Publication Details: Peer-Reviewed, Research Study Analysis
Category Topic: Coronavirus Information - Academic Publications

Page Content: Synopsis - Introduction - Main

Synopsis: A new study of people with mild COVID-19 estimates how long people are infectious and when they can safely leave isolation. The research, led by Imperial College London and published in The Lancet Respiratory Medicine Journal, is the first to unveil how long infectiousness lasts after natural COVID-19 infection in the community.

Introduction

First real-world study gives detailed new insights into when people with COVID-19 are infectious. A new study of 57 people with mild COVID-19 estimates how long people are infectious and when they can safely leave isolation.

Main Content

The research, which is led by Imperial College London and published in The Lancet Respiratory Medicine Journal, is the first to unveil how long infectiousness lasts after natural COVID-19 infection in the community. The study team conducted 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.

The findings suggest that most people who develop symptoms are not infectious before symptoms develop. However, two-thirds of cases are still infectious five days after their symptoms begin. They also suggest that while lateral flow tests do not detect the start 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 Respiratory Infections Health Protection Research Unit at Imperial, said:

"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'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 viruses (not just PCR) and daily symptom records, we could 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."

He added:

"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 generalizable 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."

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 to isolate until they are not infectious. Despite this, there is a lack of clarity on how to come out of self-isolation safely. Our study will first assess how long infectiousness lasts, using real-life evidence from naturally acquired infection. Our findings can thus inform guidance on how to end self-isolation safely."

She added:

"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 minimize contact with other people."

Most Complete Picture of the Course of Infectiousness to Date

Previous studies estimating how long someone is infectious have been a laboratory-based human challenge study(1) or have used mathematical modeling.

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(2).

Participants completed daily questionnaires about their symptoms(3) and did daily nasal and throat swabs sent to a laboratory for PCR testing. PCR-positive samples were then tested to determine if they contained an infectious virus and how infectious it 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. Still, not all were included in some analyses because some participants did not share information about their symptoms, some were not shedding culturable viruses, and some were shedding infectious viruses 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.

Real-World Timeline of Infectiousness

The study found that the overall median amount of time that people were infectious was five days.

Though 24 out of 38 people tested positive on a PCR test before they developed symptoms of COVID-19, this does not indicate infectiousness; most people only became infectious after they developed symptoms. Only one in five participants were infectious before symptom onset (7 out of 35 cases).

Although infectiousness levels reduced during infection, 22 of 34 cases continued to shed infectious virus five days after symptoms began. Eight of these people continued to shed infectious viruses for seven days.

Current NHS guidance(4) suggests that people should try to stay at home and avoid contact with others for just five days.

Safely Self-Isolating

To help understand when people may be able to leave isolation safely, the researchers compared levels of infectiousness with lateral flow test results.

They found that the sensitivity of these tests in identifying when someone was infectious was poor at the start of infection but high after peak levels of infectiousness (sensitivity of 67% vs. 92%, respectively). This suggests lateral flow tests are good at spotting when someone is no longer infectious, and testing to release people from isolation may work. Still, they are not reliable for early diagnosis unless used daily.

Based on their findings, the researchers recommend that people with COVID-19 isolate for five days after symptoms begin, then complete lateral flow tests from the sixth day. If these tests are negative two days in a row, it is safe to leave isolation. Suppose a person continues to test positive or does not have access to lateral flow devices. In that case, they should remain in isolation and minimize transmission to others, only leave on the 10th day after their symptoms begin.

Professor Lalvani said:

"Self-isolation is not necessary by law, but people who want to isolate need clear guidance on what to do. The NHS currently advises that if you test positive for COVID-19, you should try to stay at home and avoid contact with other people for five days. However, our data suggest that under a crude five-day self-isolation period, two-thirds of cases released into the community would still be infectious -- though their level of infectiousness would have substantially reduced compared to earlier in the course of their infection."

He added:

"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. We recommend that anyone exposed to the virus with symptoms be isolated for five days and uses daily lateral flow tests to leave isolation when two consecutive daily tests are negative safely."

The study did not assess the Omicron variants currently circulating. There is evidence that Omicron variants have a lower viral load and shed for less time than other variants. The researchers note that their recommendations may be cautious but still applicable if this is true.

Most participants in the study 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 at clearing the virus.

A person's infectiousness is one factor involved in the transmission, alongside behavioral and environmental factors, such as where people are mixing and if they are near one another.

COVID-19 Questions and Answers

Emily Head speaks to the authors of The Lancet Respiratory Medicine study, Dr. Seran Hakki (SH) and Professor Ajit Lalvani (AL) from Imperial's National Heart & Lung Institute, to find out more.

Q. What's new about this study?

A. (AL) Previous studies estimating how long someone is infectious have been laboratory-based human challenge studies or have used mathematical modeling.

Our real-world study involved detailed daily tests of 57 people exposed to SARS-CoV-2 in their homes to look at how much infectious virus they were shedding throughout their infection.

Q. How long am I infectious if I have COVID-19?

A. (SH) Our study found that the average amount of time that people were infectious was five days.

However, this varied from person to person.

Only one in five participants in our study were infectious before symptom onset (7 out of 35 cases), so most people only became infectious after they developed symptoms.

22 of 34 people continued to shed infectious virus five days after their symptoms began, and eight continued to shed it at seven days.

This is despite the levels of infectious virus people shed reducing over the course of infection.

Q. How long should I isolate if I have tested positive or have symptoms?

A. (SH) As per NHS guidance, if you test positive for COVID-19 or have symptoms after being in contact with someone with confirmed COVID-19, you should stay home and minimize contact with other people.

Based on our findings, we recommend that people with COVID-19 isolate for five days after symptoms begin, then use lateral flow tests to leave isolation safely. This supports the current guidance for NHS staff who test positive for COVID-19.

Q. How can I work out when it's safe to leave isolation?

A. (SH) Our study found that, while lateral flow tests are unreliable for early diagnosis (unless used daily), they are good at spotting when someone is no longer infectious, so testing to release people from isolation may work.

After five days of isolation, people can complete lateral flow tests on days six and seven.

If these tests are negative, we think it is safe to leave isolation on day seven.

If a person continues to test positive on day six or beyond or does not have access to lateral flow tests, we think they should remain in isolation until they've had two consecutive days with negative test results. Or, they can leave isolation on the 10th day after their symptoms begin. Based on the evidence available, we think it's unlikely for people to still be infectious on day 10.

Q. What about if I'm vaccinated?

A. (AL) The people in our study were infected between September 2020-March 2021 and May October 2021, meaning only 25 of 57 had received two vaccine doses.

Because of this we had limited statistical power to detect differences by vaccination status. In an exploratory analysis, we did find preliminary evidence that infectiousness declined somewhat faster in vaccinated cases, but more research is needed.

For now, we think it's best to follow the same approach if you're vaccinated.

Q. What if there's a new variant?

A. (AL) Future variants may vary, so we can't say that this isolation approach will apply to people infected with these.

Our study looked at people infected with pre-Alpha SARS-CoV-2 virus and the Alpha and Delta variants.

While our study did not assess the Omicron variants currently circulating, we believe that what it says about infectiousness and isolation may be cautious but still applicable. This is because there is some evidence that Omicron variants have a lower viral load and shed for less time than other variants.

References:

Seran Hakki, Jie Zhou, Jakob Jonnerby, Anika Singanayagam, Jack L Barnett, Kieran J Madon, Aleksandra Koycheva, Christine Kelly, Hamish Houston, Sean Nevin, Joe Fenn, Rhia Kundu, Michael A Crone, Shazaad Ahmad, Nieves Derqui-Fernandez, Emily Conibear, Paul S Freemont, Graham P Taylor, Neil Ferguson, Maria Zambon, Wendy S Barclay, Jake Dunning, Ajit Lalvani, Anjna Badhan, Robert Varro, Constanta Luca, Valerie Quinn, Jessica Cutajar, Niamh Nichols, Jessica Russell, Holly Grey, Anjeli Ketkar, Giulia Miserocchi, Chitra Tejpal, Harriet Catchpole, Koji Nixon, Berenice Di Biase, Tamara Hopewell, Janakan Sam Narean, Jada Samuel, Kristel Timcang, Eimear McDermott, Samuel Bremang, Sarah Hammett, Samuel Evetts, Alexandra Kondratiuk.

Onset and window of SARS-CoV-2 infectiousness and temporal correlation with symptom onset: a prospective, longitudinal, community cohort study. The Lancet Respiratory Medicine, 2022.

The study was funded by the National Institute for Health and Care Research.

Attribution/Source(s): This peer reviewed publication was selected for publishing by the editors of Disabled World (DW) due to its relevance to the disability community. Originally authored by Imperial College London and published on 2022/08/19, this content may have been edited for style, clarity, or brevity.