La Niña could create a spike in Covid cases by forcing people indoors to avoid the rain at the same time as the Omicron variant is spreading in Australia, say epidemiologists who are warning health authorities to delay the easing of social distancing measures.
The La Niña weather pattern means north and eastern Australia is experiencing a relatively cool, damp and stormy summer. Epidemiologists say it could drive a spike in Covid cases by making more people gather indoors and by generating higher humidity which allows virus particles to linger longer in the air.
New South Wales will relax its Covid rules from Wednesday. Unvaccinated residents will, from then, enjoy the same freedoms as those who have received their jabs.
Masks will only be required on public transport, at airports, and for unvaccinated hospitality staff. Additionally, QR-code check-ins will only be required for high-risk settings, including hospitals and aged care, while density limits will be scrapped entirely.
More than 93% of people over 16 in NSW are fully vaccinated but daily infection numbers – that had remained in the hundreds in recent weeks – are increasing. There were 260 new cases on Tuesday, then 403 on Wednesday and 420 on Thursday.
Prof Peter Collignon, an infectious diseases physician and microbiologist at the Australian National University, believes the latest uptick in cases in NSW is the result of continued rain in recent weeks associated with the La Niña event.
“One of the reasons we’re seeing more cases in Sydney recently is because of the terrible weather, we’re all indoors more [and] as soon as people congregate indoors more, surprise surprise, we see more cases,” he said. There is a lower risk of transmission outdoors due to better ventilation.
Collignon said any infection spike associated with more rain over the summer was unlikely to be as severe as in Europe and North America during their harsh winters. But it could still be noticeable and require appropriate, setting-specific precautions.
“[La Niña] may mean we see more cases this summer … when it’s miserable outside, you’re inside, and you’re often doing it with others for hours in a crowded setting with alcohol. The bigger the group, the bigger the risk,” Collignon said.
Collignon acknowledged there would be ongoing Covid transmission and he broadly supports easing restrictions in months when people are gathering outdoors – largely because of high vaccination coverage, low hospitalisation rates (there are currently 151 Covid patients in hospitals in NSW) and even lower death rates.
However, in response to the dramatic winding back of rules set for NSW next week, he said “in settings where there is more danger, where the consequences are more severe, that’s where you need to take precautions”. Collignon noted this was particularly the case in hospitals and healthcare premises.
Mary-Louise McLaws, a professor of epidemiology at the University of NSW and adviser to the World Health Organization, agrees rainy weather could increase Covid spread over summer, not only because it would force gatherings indoors, but also because of changes to the air associated with La Niña.
“Certainly with the changes to the immuno acids, there’s the potential for this virus to have more charge because particles are protected through a water molecule in the air. When they have better charge, rather than drying out, they stay in the air for a bit longer,” she said.
However, McLaws noted that if humidity gets very high, there could be too much moisture in the air, making Covid particles heavier and forcing them to the ground more quickly. She said physicists are still researching the exact impact of humidity on Covid spread.
Regardless, McLaws said the potential for greater transmission over summer, coupled with the fact that the transmissibility, virulence and vaccine-resistance of Omicron has yet to be confirmed, meant it was too soon to remove restrictions and allow unvaccinated and vaccinated people to mix unmasked in shops and hospitality venues in high densities.
“It’s definitely unwise to push ahead with the relaxations. It is so much easier to ask Australians to continue wearing a mask and to wait until we know more about Omicron, and for those who are not yet fully vaccinated to wear a mask until we know more,” she said.
“It’s too cavalier to throw away these really successful infection prevention strategies.”
McLaws said wearing masks and applying density limits was a relatively light burden compared to the harsher lockdowns in place earlier this year.
McLaws also argued lower vaccine coverage among younger, more socially mobile people, was a potential problem. “Australians are desperate to relax and enjoy themselves after a year of lockdown, and this is a potentially problematic time of the year,” she said.