Health experts broadly supported the Government's move to shorten isolation periods for Covid-19 cases but said the same change for contacts of cases could fuel the spread of the virus, Marc Daalder reports
Starting this weekend, Covid-19 cases and household contacts who test negative with a Day 7 Rapid Antigen Test (RAT) will be able to end their isolation period after just a week, rather than the current 10 days.
Covid-19 Response Minister Chris Hipkins said the changes were based on public health advice that suggested only a small number of cases might still be infectious seven days after their symptoms began.
"We remain committed to reducing the spread of Omicron to protect our health system and each other. However high case numbers and household contacts, and the current 10 day isolation requirement, is having a wider impact on many parts of our lives," he said.
"There needs to be a balance between effectively controlling the outbreak and the flow-on effect for business and essential goods and services such as transport and food supply."
Experts who spoke to the Science Media Centre said the move made sense for Covid-19 cases, but not for contacts.
"With a virus as infectious as the Omicron variant, unfortunately a high proportion of household contacts will get infected," Covid-19 Modelling Aotearoa's lead researcher Emily Harvey said.
"Whilst the majority of infections from the first case in the household will take place in the first week (as Minister Hipkins states), there is then an extended period of time (3-5 days) before someone would test positive on a RAT. This means that a large proportion of household contacts who get infected would not test positive until after Day 7. This is even before considering the possibility of chains of transmission within a household, when the household is more than two people."
Releasing contacts who end up being infectious cases into the community could prolong the outbreak.
"If we keep the 10 day isolation period, the impacts should decrease from here. However, if we increase transmission rates by shortening the isolation period, we risk creating more transmission and a longer plateau, with disruption for longer, or even a second peak, with greater disruption than we would have had if we had not reduced the isolation period."
Michael Plank, a University of Canterbury mathematics professor and a disease modeller at Te Pūnaha Matatini, said the Government should consider adding a "test to release" requirement for cases and contacts.
"Some people may be infectious for longer than seven days and it would be preferable to use a 'test to release' approach where people take a rapid antigen test on Day 6 and Day 7 and can end their isolation period if both tests are negative. This would minimise the risk of people going back into the community while still infectious, while still allowing most people to shorten their isolation period to seven days."
While PCR tests can detect viral material long after someone is no longer infectious, RATs tend to only return a positive result during a case's infectious period. This means negative rapid tests could be used to clear people for release.
"Test to release' is even more important for household contacts because they could potentially get infected part way through their isolation period," Plank said, echoing Harvey.
"For example, a household contact could still be in the incubation period on Day 3 and get a false negative on Day 7. Doing an additional rapid antigen test on Day 6 would reduce the chance of this happening."
The Government says its approach is based on those of countries like the United Kingdom and Australia, which have also shortened isolation requirements to seven days. The ACT Party's calls for it to go further point to jurisdictions like the United States, where isolation periods are just five days for Covid-19 cases.
But Harvey said most of these countries had additional requirements to end the shorter period or requirements on people after isolation ends.
In the United States, for example, cases who end their isolation period after five days are told to wear a mask when around others for another five days.
In the UK, the seven day period is combined with a test to release requirement for cases. Modelling by the UK Health Security Agency has found this adds no additional risk when compared with a 10 day isolation period - only about 5 percent of cases that were still infectious would make it into the community. Without test to release, however, that figure jumps to 16 percent. That's more than triple the current risk for cases alone.
"It is disappointing that NZ has chosen to shorten the isolation period for confirmed cases without this important safeguard. In workplaces where people work in close proximity to others, this change in rules risks increasing transmission in those workplaces and worsening staff shortages," Harvey said.
"The evidence-based recommendation is to follow the UK guidance if you can (two negative RATS at least 24 hrs apart to leave isolation). This however requires people to have access to more RATs than the Government provides in its current rules, and will only be available as an option to those who can afford to purchase RATs from retailers."
