Of the many different strategies countries are adopting to combat Covid-19, an increasing number include rapid antigen testing. Austria recently committed to the purchase of rapid tests, while in Canada a program was launched that aims to distribute rapid tests for workplace screening. Rapid tests can be administered free of charge and can be rapidly deployed. Yet, while rapid antigen testing could be an important step, a new study confirms that without a comprehensive testing regimen that includes PCR follow-up and contact tracing, rapid tests fall short as a method of Covid-19 control.
A study conducted as part of national Covid-19 surveillance in England looked into the performance of antigen lateral flow devices. A lateral flow device is used to process a nose and throat swab sample for testing. When a person is infected with Covid-19, the lateral flow device recognizes a Covid-19 antigen that is generated. A coloured strip on the lateral flow devices will appear to show a positive result if this antigen is present. Data from community and hospital PCR testing in England was linked with national contact tracing data between September 1st 2020 and February 28th 2021.
The data analyzed comes from two main sources: PCR-positive index cases and their close contacts. Index cases were defined as PCR-positive individuals with a community-based test from three national testing facilities which reported Ct values indicating a viral load. The Ct value, which stands for cycle threshold, represents the number of PCR cycles after which a virus can be detected. Contacts of index cases were defined as individuals notified to the national tracing service who were in close proximity to PCR-positive cases 48 hours before their symptom onset to ten days afterwards. Within 10 days of their test, about a million index cases had nearly three million contacts reported, of which 21 percent tested PCR positive.
The next step was running simulations that test what proportion of positive contact cases antigen lateral flow devices would detect. The study focused on viral loads of contact cases as antigen lateral flow device sensitivity differs by viral load. A positive or negative lateral flow device outcome was simulated for each source case by drawing at random from the likelihood of a positive lateral flow device based on the source case’s Ct value. Each simulation was carried out 1000 times. Results indicated that the rapid antigen tests can detect 83 percent to 89.5 percent of infections leading to onward transmission.
A limitation to this study is that the calculated benefit is particular to demographic and pandemic specifics. Nonetheless the do show that rapid testing can and does work when coupled with other mitigation measures. Rapid antigen tests are not accurate enough to be used as a standalone, and should be used only if the necessary follow-up is available.
Singapore has had an effective test, trace, and isolate system since the pandemic started. With the country’s recent surge in cases, the government has been rolling out rapid antigen tests to speed up contact tracing. Following this approach, I would like to see more countries introduce a program called test, trace, isolate, prevent.
Rapid antigen tests can be used for weekly testing in order to detect the most infectious cases and to prevent onward transmission. Positive cases should be confirmed using a PCR test and then isolated for fourteen days. Contact cases should then be traced and quarantined for at least ten days, and may only be allowed out of isolation if they are PCR-negative.
Rapid antigen tests are not reliable enough to replace PCR testing, but are an essential preventative tool that could give us the upper hand against Covid-19. If governments have the resources to adopt a test, trace, isolate, prevent program, lateral flow devices offer an additional line of defense which allows for people to go outside more safely.