People may test positive for SARS-CoV-2 in a rectal swab despite testing negative in a throat swab

Breaking new ground 15. feb 2022 2 min Associate Professor and Senior Researcher Suzette Sørensen Written by Kristian Sjøgren

People who test negative for SARS-CoV-2 in a throat swab could still have COVID-19. New research shows that the virus can colonise a person’s gut.

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New research shows that more than half the people who test positive for SARS-CoV-2 using a throat swab also have the virus in their gut. In addition, after some time some people may test negative for SARS-CoV-2 in a throat swab but positive in a rectal swab.

This suggests that people who test negative may still infect other people through their faeces, especially children or people in nursing homes, who often receive help to change diapers or incontinence pads or go to the toilet.

The discovery shows a potential route of transmission that not many countries have considered in their COVID-19 preparedness.

“We talk a lot about how we can prevent the transmission of SARS-CoV-2, so the fact that it is also present in the gut of some individuals may potentially affect not only the risk of transmission but also the disease trajectory,” explains a researcher behind the study, Suzette Sørensen, Associate Professor, Aalborg University and Senior Researcher, North Denmark Regional Hospital, Hjørring.

The research has been published in Frontiers in Medicine.

More than half tested positive in rectal swabs

The researchers examined the disease trajectory of 42 people hospitalised with COVID-19 and recently testing positive in a throat swab. Throat and rectal swabs were then collected from all participants.

Of the 42 participants who tested positive in a throat swab, 23 (54%) tested positive in the rectal swabs.

Suzette Sørensen says that this is comparable with data from studies in China.

Some participants tested negative for both swabs at the same time. Some tested negative first in the rectal swab and then the throat swab. Most interesting, however, is that some participants first tested negative in the throat swab and did not test negative in the rectal swab until many days later.

One person continued to test positive in the rectal swab for 29 days after a negative throat swab.

“This might lead to some transmission scenarios in which a person is considered negative for SARS-CoV-2 while being infected. For example, one can imagine situations in which infected and uninfected people are placed in the same room in a hospital without them or the personnel knowing that some are infected,” says Suzette Sørensen.

Suzette Sørensen also says that they found no difference in the severity of COVID-19 between those who tested positive and negative in rectal swabs.

One person with SARS infected an entire apartment building in 2003

One can imagine various scenarios in which infection can be transmitted from one person who would test positive in a rectal swab to another person.

For example, a child in childcare who needs a diaper changed or needs help on the toilet might infect an early childhood educator or other children.

The same problem can arise in nursing homes when people need help to go to the toilet or change an incontinence pad.

In both situations, a person may test negative in a throat swab but transmit the infection through faeces.

However, studies of this potential route of transmission are not unanimous, with some suggesting that it is possible and others not.

Previous coronavirus epidemics, such as the 2003 SARS outbreak in Asia, provide some evidence that coronavirus can be transmitted through faeces.

In the 2003 outbreak, one person with SARS-related diarrhoea supposedly infected a large number of people in an apartment building through the pipe system

“We do not know whether this applies to SARS-CoV-2, but examining this is sensible, so we have a full overview of how viruses can be transmitted. This may have implications for how future epidemics are combatted,” explains Suzette Sørensen.

Transmission may be greater in other countries

Fortunately, Denmark has relatively good regulatory control of hygiene and plumbing installations, so SARS-CoV-2 is unlikely to be transmitted to an entire apartment block.

This may differ in other countries, however, and according to Suzette Sørensen, carrying out further research on the potential for transmission when a person tests positive for SARS-CoV-2 in a rectal swab may therefore be important.

If the risk of transmission turns out to be very high, this should be considered when planning interventions to stop transmission.

In Denmark, the most relevant perspective of the new research is more closely related to the significance for the person infected. For example, whether testing positive for SARS-CoV-2 in a rectal swab for a long time after testing negative in a throat swab affects the risk of long COVID is still unclear.

“This may be worth examining because it is a crucial outcome,” concludes Suzette Sørensen.

The prevalence and clinical implications of rectal SARS-CoV-2 shedding in Danish COVID-19 patients and the general population” has been published in Frontiers in Medicine. In 2020, the Novo Nordisk Foundation awarded a grant to Suzette Sørensen for the project Evaluating the Prevalence and Burden of Rectal SARS-CoV-2 Shedding in the Danish Population.

The Department of Clinical Medicine was established on January 1st, 2013. The department has its physical location at Aalborg University Hospital wher...

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