It appears New Zealand wiped out COVID-19 just in time, with a new study suggesting that it spreads more easily in winter weather - just like influenza and other seasonal respiratory viruses.
Scientists in the US looked at the first eight cities around the world to suffer widespread outbreaks of the disease - Wuhan, China; Tokyo, Japan; Daegu, South Korea; Qom, Iran; Milan, Italy; Paris, France; Seattle, US; and Madrid, Spain.
All were located on a "narrow band" of the planet, between 30 degrees and 50 degrees north, all were cold at the time - with mean temperatures between 5C and 11C - and all had low humidity.
"Because of geographic proximity and substantial travel connections, epidemiological modeling of the epicentre estimated that regions in southeast Asia, specifically Bangkok, would follow Wuhan in the epidemic," the new study, published in journal JAMA Network Open, reads.
"However, in reality, the number of cases in the subsequent days in these regions remained low as the epicentre shifted to other countries in Asia, Europe, and North America."
Europe was the first to be hard-hit, particularly Italy. The first US deaths were reported in Seattle's state of Washington in February, before it really took off in the larger metropolis of New York - which at 40 degrees north, is smack-bang in the middle of the zone the scientists identified.
The study looked at the spread of the disease up until March 10. New York's outbreak didn't start until the following week.
"During the same period, COVID-19 failed to spread substantially to countries immediately north... and south of China," the study said, noting at the time Moscow to the north and Bangkok to the south had reported no deaths.
The findings are at odds with Chinese research released in April, which found no correlation between temperature and humidity and the virus' spread within China.
"The result of our analysis suggested that ambient temperature has no significant impact on the transmission ability of SARS-CoV-2," scientists from Shanghai's Fudan University concluded. "It is premature to count on warmer weather to control COVID-19, and relying on seasonality to curb this pandemic can be a dangerous line of thought."
But it backs up Australian research released earlier this month, which found new cases are more likely in areas with low humidity.
Since early March, the virus has spread much more widely than just the cities looked at in the new study - including in hotter countries such as Brazil.
With no one immune to the new disease, it could be that Brazil's relative warmth and high humidity is preventing a much worse outbreak - but the authors say climate is just one factor that influences a virus' transmission.
"Other potential factors that influence transmission (eg, other climate factors, public health interventions, travel, population density, air pollution, population demographic characteristics, viral factors) were not included in this study."
Brazil is cooler now than it was during the time period the study looked at. Moscow was on average below zero in February, but in April - when its case numbers started rapidly rising - its average temperature was 11C - within the range the researchers noted. New York's average temperature also entered that range between February and March.
It's yet to be known if SARS-CoV-2 - the virus which causes COVID-19 - will become seasonal or endemic (always present). A third option - eradication - appears to be looking increasingly unlikely, at least until a vaccine is available.
"The spread of the SARS-CoV-2 virus in the upcoming years could follow different patterns; it could prevail at low levels or cause several seasonal peaks in tropical regions like influenza, cause outbreaks in the southern hemisphere at the same time, and begin to rise again in late fall and winter in temperate regions in the upcoming year.
"Another possibility is that, combined with intensive public health efforts, it will not be able to sustain itself in the summer in the tropics and southern hemisphere and disappear, just as SARS-CoV did in 2003; however, the ever-increasing number of cases worldwide make this increasingly less likely."
New Zealand has no known cases of the virus at present, and has closed the borders to keep it out.
