Let virus biology direct COVID-testing policy

There are many interconnected parts to our COVID-19 response. Our public health contact tracing capability has been heavily relied on to link cases enabling us to understand the epidemiology of spread. Our laboratory network has responded by having the capacity of going from a few thousand tests a day to more than 30,000 tests a day in surge testing in response to the latest delta outbreak. Wastewater testing gives an indication of the presence of community cases and molecular typing of isolates confirms subtype and links between cases.

Testing border and MIQ staff is also an important component in defending our border. Understanding the replication cycle of SARS-CoV-2 (SARS-2), the virus causing COVID-19, is critical in deciding on testing frequency. Earlier variants of SARS-2 took around 5 days to become infectious. Testing has been recommended at weekly or fortnightly intervals. These intervals however are not based on SARS-2 replication, but rather a compromise between biology and how far we can impose nasopharyngeal testing on workers. If a simple mouth swab had been a sensitive way to detect the virus we would have been recommending testing every four days or so. The newly arrived delta variant replicates quicker and reaches higher infectivity a couple of days earlier than previous strains. For those at risk of frequent SARS-2 exposure in the workplace this speed of replication mandates that more frequent testing is undertaken. Frequent testing is the only way to detect infections early. The sooner appropriate isolation and contact testing practices are deployed the better.

Our understanding of SARS-2 continues to evolve along with the virus. Recent observations from American researchers reveal additional insights into SARS-CoV-2 biology. In these longitudinal studies paired nasal and saliva samples were obtained from recently infected people. In vaccinated people, including those receiving the Pfizer vaccine used in New Zealand, some breakthrough infections were only detectable in saliva. Importantly, in most people the viral load peaked a day earlier in saliva. While these studies remain to be fully published, they provide important information to inform the screening policy for border and MIQ workers.

The biology of SARS-2 means those needing frequent workplace testing should undergo testing every two days. Saliva is the specimen that allows this testing frequency to be acceptable to these important work groups. Maintaining a safe border will continue to be an important component in our COVID-19 protection plan for some time to come. Testing policy must be informed by understanding the biology of the virus. Anything less places our border control at risk.

​Dr Arthur Morris.

Arthur Morris is a pathologist, clinical microbiologist, with a long-term interest in infection prevention and control. He is the supervising pathologist for IGENZ.

In January 2021, IGENZ Ltd, a contracted testing laboratory, achieved IANZ accreditation for saliva testing using the Rako Science methodology, based on validation work that used a set of contemporaneously collected paired saliva and nasopharyngeal samples.

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