were held in Liverpool in May at the Circus nightclub and the Sefton Park Pilot Music Festival. The UK’s Events Research Programme included these events, which gave audiences a taste of the old norm of gathering without social distancing and face coverings.
Participants had to be able to demonstrate that they did not have COVID-19 by having a negative rapid test of lateral flow. All COVID-19 testing, including the lateral flow test, can miss some cases. There was a possibility that some attendees could have been infected and infected other people. It’s crucial to understand the risk if similar events are to be held again soon.
We have found this out through testing and statistics. In the 36 hours leading up to an event, each attendee completed their lateral flow tests. Then, they were asked to do a different test (a test using a home kit that was sent to a laboratory for analysis) both the day of the event and five days after.
The tests are easy to perform. The tests are quick and give an immediate yes or no response on whether someone is COVID-19 positive. They work a little like a pregnancy check.
Despite the fact that everyone tested negative, lateral flow testing will still miss some infections. Ken Biggs/Alamy Stock Photo
The PCR tests were more sophisticated. The PCR tests did not only show whether someone was positive but also provided a number known as the ” Cycle Threshold (Ct), which reflects the amount of coronavirus in the test swab of the individual. The lower the Ct, the greater the amount of virus in the body. This means that the person is more likely to be infectious. Negative PCR is when Ct exceeds a certain threshold.
Calculating the duration of an infection
These tests can be used to determine how many people were infected at the event or nearby. You can do this by first using the follow-up test to determine who has an infection soon after. You can then estimate the time that any individuals who test positive caught the virus, either before, during, or even after the event.
You need to know the dynamics of COVID-19 in order to determine when the virus was acquired. These are fairly simple. COVID-19 enters the body in two phases. The first is an exponential rise of the virus as it gains hold.
We can calculate the average COVID-19 trajectory using existing data and analysis from a large number of previous infections. The red line on the graph below shows the average amount of virus that is in the body during an illness. This number, called the Ct, can be calculated by using existing data.
This is a basic illustration showing how the virus amount in the body changes (in red) during a COVID-19 illness. Author provided
The Ct value from a positive PCR result could be indicative of one of two infection stages – either the infection is increasing or declining.
If, for example, the Ct number of the person is C 0 in the graph, their test could have been taken at either point A (on the time axis X) or B. We can use the chart to estimate their Ct levels at the time of the event. It also gives us an idea of the extent of their infection and when they were infected.
Which is the correct answer? We know that the person must have had a negative result 36 hours prior to the event in order to attend. When people have a small amount of virus, lateral flow tests can be falsely positive. However, they are less likely to produce a false negative when someone has a large viral load.
If one of two possible levels of viral infection at the time the event occurred – C A or C B indicates a higher viral load than the other, then the case of a high viral burden can be discounted. We can also use historical data to calculate the probability that a test will return a negative when the Ct numbers are C A and C B. This allows us to determine which estimate of when they became infected was more likely.
For each person who tested positive in a PCR after the event, we can estimate when they first caught COVID-19. If the estimated time of infection is before the event, we assume they caught the virus somewhere shortly before, and their lateral-flow test failed to detect their disease. This could be because they are still early in the infection process and have low levels of virus. We can estimate their viral load and how infectious they were during the event.
Those who appear to have been infected at the event or shortly afterward were actually there. Then, we can compare the two groups, the infected and the infectious, to determine how much spread occurred and how dangerous the event was.
Using these techniques, we were able to confirm that the Liverpool events did not have any impact on the Liverpool region. By assessing the safety of these events, we only informed policymakers. The task was to determine if the identified risk was low enough for us to resume our normal lives.