The virus that causes COVID-19 is known to spread in clusters. Identifying these sources of infection is key to reducing spread, unfortunately, it is often not possible to know where people got infected particularly when there is significant spread of the virus in the community. There are, however, several tools that can provide vital clues. One of the best is data on what are known as ‘common exposures’.
These are the locations or events that a number of different people who tested positive for COVID-19 visited in the same period of time in the days before they tested positive – the activities they have in common.
Sometimes the links are not hugely informative. If two people both visited the same supermarket but that is the only supermarket in the town then it is likely to be a coincidence.
But if multiple people all test positive having been to the same gym, at the same time, on the same day, then that would be well worth investigating. Common exposures identify links between cases and possible outbreaks, but they do not provide definitive information about how transmission occurred. Further investigations are needed. Local authorities and local health protection teams review the daily alerts and undertake further investigations as needed.
Of course, it is crucial to weigh up information from different sources, including local knowledge and the evidence that we have from our surveillance, outbreak investigations and research studies on risk factors for transmission of the virus.
What might appear like a workplace outbreak from the common exposure data could, for example, turn out to be people who each got infected because they share a home or travel together to work.
Transmission can occur in almost any setting. It is the combination of environmental factors and human factors such as the degree of contact between people that lead to transmission of the virus.
To assess the risk of transmission we need to consider:
- Contact patterns, such as proximity and duration of contact between people and the number of contacts.
- Environmental factors, including ventilation, hygiene practice and the likelihood of the activity generating droplets and aerosols, for example singing or loud speaking
- Socioeconomic factors, for instance household overcrowding.
The highest risks occur when multiple factors combine, and it is important to act to tackle all these risk factors. Implementing the measures that are known to be effective in reducing risk such social distancing measures, reducing sharing of facilities or ensuring good ventilation Is key although it will be more challenging in some settings compared to others. The effectiveness of the mitigation measures is one of the factors considered when investigating the possible source of an outbreak.
The common exposure alerts are one of the key tools we use to help identify chains of transmission. Breaking those chains is vital in our fight against COVID-19. We use the information that we collect during contact tracing to identify these links between cases. This is one of the reasons why it is so important that if you are contacted by NHS Test and Trace you respond promptly and provide us with as much information as you can. Your action is key in helping us to identify and control sources of infection.
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Contributor: Isabel Oliver