QMRA Data Experts
- Dec 24, 2021
- 3 min read

To open up or not: Is a summer Christmas a gift for New Zealanders?

Pundits have long warned that the easing of New Zealand’s Covid-19 rules could lead to a summer crisis[1]. But despite these warnings, the New Zealand government seems determined to move to open borders, whether in Auckland or elsewhere in the country, to allow Christmas-related travel[2]. Further easing of international borders is expected early next year, although some are cynical about the delay in opening up at least the trans-Tasman bubble, since no fully vaccinated travellers from Australia to New Zealand in the last three months have tested positive for Covid-19 after arrival[3]. On top of this pressure, we have to deal with the newly emerged Omicron variant, with its many spike protein mutations that may make it more readily transmissible and better at escaping the immune system[4]. Apparently, this variant will test our vaccination strategy. Omicron is already present in many states in the US[5], but much remains to be learned about this new Covid-19 variant[6], which is already spreading in Australia[7]. How will all this play out this Christmas for Kiwis?

As the length of time the virus can survive on surfaces depends on many factors including temperature, humidity and UV or sunlight[8], New Zealanders may get some respite, as our Christmas coincides with high summer temperatures. In Japan, China, Spain, Brazil and the US, studies have shown that environmental temperature is related to the number of recorded cases. For instance, one study showed a possible association between low temperatures and increased Covid-19 infection[9]. Another study also indicated that a one degree rise in temperature was associated with a small reduction in cases per 100,000 population.[10]. An analysis of climate data shows a significant reduction in case rates with average maximum temperatures above 22.5 degrees Celsius. Another study[11] found that low air temperatures helped to drive the Covid-19 outbreak in China. Further investigation found that the transmission rate decreased as temperatures increased, reducing the size of the outbreak. Temperature seems to be the main climatic predictor for the incidence of Covid-19, so the recent drop in cases in New Zealand as we head into the hot summer days is not surprising. In recent days, New Zealand has experienced high daily temperatures, with severe heat waves predicted[12] in some instances[13]. Already, it is widely anticipated that New Zealanders are in for a hot, humid summer[14]. Although there is no New Zealand-specific modelling investigating the Covid-19 infection rate–temperature linkage, based on the results from other studies in Japan, China, Spain, Brazil and the US, it is anticipated that the infection rate and size of the outbreak in New Zealand will reduce further over the hot summer months. While this may be contested, depending on the number of cases we see in the coming days, what is clear is that the number of cases will be lower than if we were entering a cold season. Summer therefore provides a window of opportunity for government and other stakeholders in the health sector to prepare for the uncertainties that lie ahead once the temperature plummets again. Hopefully by then New Zealand’s vaccination rates, based on the existing mRNA vaccine and other non-mRNA options, will be higher than they now are.

Because the risk of Covid-19 illness is a product of hazard, chance and exposure, in addition to promoting the uptake of vaccination, which reduces the chances of infection and the severity of illness among at-risk and under-represented groups, I continue to support the other existing nation-wide preventive efforts. These focus on mask wearing, physical distancing, hand hygiene, contact tracing, and case isolation, which limit exposure and reduce the chances of spread. On top of this we have the weather in our favour, with our hot summer being a gift for New Zealanders this Christmas.

Since high summer temperatures most likely mean that infection rates will decrease, should we fling open the country’s doors to everyone who wants to come to New Zealand? The answer is not clear-cut. Given that we are close to achieving the aim of having 90 per cent of all eligible New Zealanders fully vaccinated by mid-December[15], it makes sense to open up gradually, firstly to fully vaccinated individuals wanting to return to NZ (which is exactly what the government is doing). A careful approach gives the state and other relevant stakeholders ample time to study the emerging cases in summer, The case numbers then would then inform a staged opening over time from 30 April onwards for all other fully vaccinated individuals worldwide. This approach by the government is both logical and robust from a scientific viewpoint.