As Covid-19 case numbers have continued to rise following the arrival of the Omicron variant alongside the easing of restrictions in Australia, the focus has begun to turn to the positivity testing rate.
In New South Wales, where the rise in cases has been steepest, the infection rate as a proportion of the population is now one of the highest in the world. By New Year’s Eve the rate of infection was about 2.59 per 1,000 people, higher than the United States.
As cases have risen, so too has the positivity rate, which by Sunday had reached 20% in NSW. Other states have also experienced a steady increase in the ratio of positive cases. In Tasmania it reached 26% on Sunday, Victoria’s was 14.8% and Queensland’s 10.5%.
So what is the positivity rate? And what does it tell us about the spread of the virus as evidence of Omicron’s milder severity continues to mount and governments try to shift the focus away from case numbers?
What is the positivity rate?
The positivity rate of infection is the number of positive cases of Covid as a proportion of the number of PCR tests carried out.
For example, a positivity rate of 20% in NSW means one in five people whose tests were processed in the reporting period had the virus.
The figure doesn’t tell us how many cases are circulating in the community, but it can help to understand how many of the likely cases are being picked up. That, according to Alexandra Martiniuk, a professor of epidemiology at the University of Sydney, was particularly true at the beginning of the pandemic.
“In general, the lower the positivity level, the more likely we think we’re catching, if not all, then most of the cases in the community,” she said.
“As it rises you think we’re likely missing cases, and the more it goes up we’re likely to be missing more and more.”
Why is it rising so quickly?
Martiniuk says that throughout the Covid pandemic, the reliability of the positivity rate has been affected by testing levels, and the policies dictating who gets tested and who doesn’t.
She uses the example of Malawi, where she previously worked as a representative for the World Health Organization. Throughout the pandemic that country did very little population testing, limiting it to people who presented at emergency departments.
In Australia, policy changes may be affecting the positivity rate. Recent changes to Covid-19 testing policies have seen governments ask people not to get tested unless they are symptomatic or fit a new and narrower definition of a close contact.
As test numbers begin to fall, as they did on Sunday, it’s likely that the positivity rate will increase.
“It’s like we had this bigger bag which contained all the people we have now, but also a lot of other people. People who might have been a casual contact, or who fit the old definition of a close contact, or who might have had very mild symptoms,” she said.
“So now our bag is smaller so that we just have those symptomatic people and the very close contacts. So we may be missing those other people.”
The positivity rate can also be affected by other things, such as the Christmas and new year holiday period. While the NSW positivity rate was 20% on Sunday, the seven-day average was about 10%.
Does it matter?
Certainly for scientists trying to understand how the virus is spreading, it is a figure worth watching.
As Martiniuk puts it, for people seeking to respond to the pandemic, data “is like the flight path”.
“Without it you’re flying blind.”
And while changes to testing may affect the number of cases being caught, she said it was also important to recognise the need to focus resources.
“I’m not so silly as to think we’re not in the situation that we’re in,” she said.
“I think epidemiologists recognise times are changing and they need to and it’s both good and bad at the same time. We are having a resource issue and there’s no way we could PCR test everyone with Omicron.
“PCRs need to get turned around faster because a lot of the treatments are needed in the first five days, so particularly vulnerable people need to know their status.”
So as the focus of the pandemic shifts so that case numbers are no longer the focus of government, is it still relevant?
As the positivity rate in Australia has climbed, some have pointed to the World Health Organization’s declaration that a positivity rate of 5% and under is evidence of spread that is under control.
But that declaration was made towards the beginning of the pandemic in May 2020. A that point, governments were still seeking to control the virus, while Australia was trying to suppress it, or in some states, eradicate it.
“Test positivity still has value, as in the metric is still valuable, but the benchmark of 5%, I’m not sure,” Martiniuk said.
“I think obviously if a country is at 5% that’s great, it is a good indicator that their local pandemic is under control, but whether it’s achievable for most countries to aim for that at this point is another question.”
However that doesn’t mean people should believe the pandemic is over.
“We still need to test and we still need to go back to that 2020 thinking of bending the curve so if someone is really sick they can get the care the need. I think that message has been lost a little bit.
“We still need testing to stop those chains of transmission even though it does feel overwhelming.”
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