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Reinfection is set to rise. (Image: NIAID, CC BY 2.0, additional design by Tina Tiller)

SocietyJuly 7, 2022

Officially, just 1% have caught Covid twice. Why that’s an undercount – and set to surge

Covid-19 virus particles
Reinfection is set to rise. (Image: NIAID, CC BY 2.0, additional design by Tina Tiller)

The way cases are counted in New Zealand is changing, and so is hospitalisation reporting – a total of 15,000 Covid hospitalisations across the pandemic becomes 8,490 hospitalised ‘from’ Covid.

New data shows a tiny proportion of Covid cases officially recorded in New Zealand have been classed as reinfections, though that number is thought to underestimate how many have caught the virus multiple times and is set to climb substantially as New Zealand enters its winter wave. It comes as the Ministry of Health recalibrates the way it reports reinfections, as well as changing the way hospitalisations are accounted for, to distinguish between those hospitalised with Covid as an underlying or contributing factor and those who have tested positive but are in hospital for reasons other than Covid.

The reinfection peril

About 1% of the Covid cases reported in New Zealand over the course of the pandemic have been recorded as reinfections, while fewer than 200 people have registered a triple infection. That number is very low compared to other parts of the world, reflecting the country’s closed borders for much of the crisis. The winter wave, which is increasingly powered by new subvariants better equipped to defy the immunity from previous infections, means those numbers will surge, while public health systems and messaging must accordingly race to “catch up”, an expert has warned.

Data from the Ministry of Health provided to The Spinoff shows that of the total 1,403,073 Covid-19 cases reported as of today, about 1%, or 14,010, are reinfections. Of those, 9,573 were detected between 29 and 90 days after a previous positive test. 

In today’s update, 351 of 11,084 newly reported cases (just over 3%) were reinfections. Of those, 112 were reinfections confirmed between 29 and 90 days since the previous positive test.

There are 183 people who have caught Covid three times or more, having recorded positive tests separated by between 29 days and 90 days. That amounts to about one in every 10,000. “This figure needed to be treated with caution,” a ministry spokesperson told The Spinoff, “as the small number involved means any over or under-reporting could have a significant impact on the reinfection rate.”

Dr Amanda Kvalsvig, an epidemiologist at the University of Otago in Wellington, stressed that the risks of reinfection were relatively new to New Zealand – and rapidly growing. “Until this year Covid-19 reinfections haven’t been a concern in New Zealand,” she told The Spinoff. “That’s partly because pre-omicron variants tended to generate good protection against subsequent infection, but also very simply because infection rates in NZ were so low that the chances of being exposed and re-exposed were far lower than elsewhere.”

Now, she said, “all of that has changed”, meaning “our surveillance systems and public health messaging will need to catch up”. 

Even the true number, said Kvalsvig, would be significantly higher. “Almost certainly, known reinfections are a substantial underestimate of the true number. We know this because testing policy is a gateway factor in counting cases and until recently, New Zealanders were being advised not to test for 90 days after an initial infection and were being told that infection would deliver lasting protection.” 

‘Our public health messaging will need to catch up‘: Dr Amanda Kvalsvig from the University of Otago in Wellington

It underscored the importance of a prevalence study. “Until we have a population survey up and running we won’t have a handle on the true reinfection risk,” she said.

Why was such a study important? “The key here is that absence of evidence isn’t evidence of absence. We have more than enough data from overseas to know that reinfections with later omicron variants are common. That knowledge should shape New Zealanders’ behaviour through winter 2022 in terms of staying home when unwell, testing when symptomatic or before going into a large gathering, wearing a mask in crowded indoor places, and optimising indoor air quality. It’s sensible not to keep rolling the dice on this potentially serious infection.”

Last week the government cut by two-thirds the period after which someone who is infected with Covid is exempt from testing if symptomatic, from 90 to 29 days. That change has now been reflected in reporting by the Ministry of Health, which announced today the addition of 6,931 “additional previous cases arising from positive Covid-19 test results reported between 29 and 90 days after a previous positive Covid-19 test”. 

The force of the winter Covid wave, which has seen case numbers and hospitalisations leap to levels not seen for months and predictions that we could soon return to the peaks of March, is driven by a mix of seasonal impacts, waning immunity from both vaccination and infections, and new subvariants that have proved much more adept at dodging human defences.

“As immunity from the vaccine and a previous Covid-19 infection wanes reinfections become more likely,” said a ministry spokesperson. “Reinfection is more common with new variants and subvariants that are better at evading the immune system. Prior immunity may be less effective at providing full protection against infection with new variants and subvariants.”

The best protection remains ensuring vaccination is up to date, as well as wearing a mask indoors and avoiding crowded or poorly ventilated environments. 

“For most people illness caused by reinfection is likely to be no more severe than a first infection, but they can experience different symptoms,” said the spokesperson. “Evidence on reinfections is evolving rapidly. We are constantly monitoring what’s happening internationally and updating our guidance to ensure it is in line with the latest public health advice.”

New reporting on hospitalisations: ‘from’ vs ‘with’

As well as changing its reporting approach to include more information about reinfections, the Ministry of Health has revised the way it details hospitalisations to show “where Covid-19 is an underlying or contributing factor, and clearly distinguishing this from when a person with Covid-19 may have been hospitalised for non-Covid-19 reasons”.

This reveals that since the start of the pandemic 8,490 people have been hospitalised from Covid-19, including 305 that have required intensive care. The previous total, tallying all people with Covid-19 who had been hospitalised, was just shy of 15,000. 

Explaining the change in methodology, a ministry release stated: “Our response to both the current and any future outbreaks depends on a strong understanding of the burden on Covid-19 on our communities, especially when infection leads to severe illness. Over recent weeks, the Ministry has been working on expanding what we know about hospitalisation.”

The ministry also provided the following details: 

  • 1,878 – just over one in five – of those hospitalised from Covid-19 spent less than 24 hours in hospital.
  • The median hospital stay from Covid-19 is three days.
  • Among people over the age of 20, the Covid hospitalisation rate for those who are unvaccinated is nearly six times higher than for those who have been boosted.
    (“As our data quality improves over time, this gap between hospitalisation rates of vaccinated and unvaccinated increases.”)
  • For unvaccinated people hospitalised due to Covid-19, the rate at which ICU care is required is roughly double the rate for those who are boosted. 
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