Rapid antigen tests offer a way to both reduce the amount of time most cases spend isolating and reduce the percentage of cases who are still infectious when they end isolation, Marc Daalder reports
Analysis: A new paper detailing how the average Covid-19 isolation period can be shortened while still reducing the risk of people returning to work infectious has reignited the political debate over isolation timeframes.
The National Party says isolation for cases should be shortened to five days. ACT wants to go further, down to a minimum of three days plus a negative test to be freed. Meanwhile, Labour and the Greens say any potential reduction in isolation periods should be considered carefully, because it might increase the number of infectious people in the community.
None of the parties engaged with the actual scenarios examined by Covid-19 Modelling Aotearoa in its study last week.
That modelling found, as expected, that a reduction in isolation periods without any other changes could be expected to increase the number of Covid-19 cases who are still infectious when they return to work and socialising.
Even under the current, blanket seven-day isolation period, between 15 and 41 percent of cases are contagious when they leave isolation.
Dropping down to five days could be expected to increase that to between 30 and 62 percent.
Test to release
The problem here is that not everyone is infectious for the same amount of time. Many will no longer be contagious just a handful of days after they first test positive or get symptoms. Others are able to infect people for 10 or more days.
But rapid antigen tests offer a way to both reduce the amount of time most cases spend isolating and reduce the percentage of cases who are still infectious when they end isolation.
Unlike the old PCR tests, which would come back positive if they detected even dead virus, rapid tests are closely aligned with infectiousness. Though they can take a couple of days after the start of an infectious period to begin returning positive results, they tend to go negative within a day or so of a case's infectious period ending.
Requiring Covid-19 cases to test negative to exit isolation can therefore individualise isolation periods, allowing those who are no longer infectious to go free before the current seven-day requirement while keeping the most infectious people in isolation for a bit longer.
The modelling found that reducing the minimum isolation period to five days but requiring two negative tests in a row to end isolation would both cut the percentage of cases released while still infectious (from 14.6 to 9.1 percent) and reduce the average number of days in isolation (from 7.5 to 6.9).
If the minimum isolation timeframe was kept at seven days and just a single negative test was required to leave, then the percent of cases still infectious at the end of their isolation would similarly drop to 9.3 percent, although the average isolation period would rise slightly to 7.8 days.
The findings create the potential for a win-win, cutting both onwards Covid-19 transmission and time in isolation.
They're also based on relatively conservative assumptions about the efficacy of rapid tests and the length of infectious periods. The test to release policy has even greater benefits if people are infectious longer than expected. When modelled at test efficacy of 95 percent, the proportion of cases still infectious at the end of isolation drops to 5.8 percent.
Most of these come at the tail end of the isolation period. Covid-19 Modelling Aotearoa's scenarios assumed everyone is freed from isolation on the 10th day, even if they're still testing positive. Around 4 or 5 percent of cases will still be infectious after 10 days.
That underscores the need for additional caution, the study's lead author Emily Harvey has told Newsroom in the past. She said anyone leaving isolation should be extra careful for a few days, including wearing a mask and avoiding high-risk situations.
The Ministry of Health has endorsed this message in recent days, with the Unite Against Covid-19 campaign encouraging cases to "wear a mask when you're out and about" after recovering.
Study ignored by politicians
Even though the modelling study offers a win-win policy, no political parties have engaged with these findings.
National still backs a flat five-day isolation period, with their Covid-19 response spokesperson Chris Bishop telling Newsroom the party would look into a test to release policy but wouldn't commit to it yet.
While this policy would cut the number of days in isolation down to 5.5, it would more than double the percentage of cases still infectious when their isolation ends.
In a press release, the ACT Party said it would implement Singapore's current policy of a minimum of three days and a test to release requirement. That scenario wasn't modelled by the researchers so it's unclear whether it delivers the same win-win benefits.
Singapore also only starts counting isolation from the date of positive test, which in New Zealand generally comes two days after symptom onset.
Labour and the Greens, meanwhile, have expressed doubt about any reduction in isolation periods but haven't engaged with the possibility of a test to release requirement.
Isolation settings are expected to be reviewed in a couple of weeks, as part of monthly reviews of Covid-19 settings that have happened since the end of the first Omicron wave.
In a statement to Newsroom, Covid-19 Response Minister Ayesha Verrall said she was unable to say whether test to release would be examined as part of the forthcoming review.
"As the discussions on future settings are underway so I am unable to provide you with the detail you seek. We are still urging that Kiwis get vaccinated and boosted, wear masks in indoor areas outside the home, get tested and stay home when you are unwell," she said.
"Our decisions on our COVID-19 settings are based on advice from the Ministry of Health and are regularly reviewed by public health officials. These reviews look at a range of factors – not just case numbers – including scenario modelling, public health considerations and the current capacity of the health sector. The reviews also ensure that the settings are appropriate and proportionate."
