Most respiratory viruses are seasonal, with outbreaks in the fall, winter and spring that diminish when weather turns warmer, according to the Centers for Disease Control and Prevention.
There remains the hope — coupled with a great deal of uncertainty — that SARS-CoV-2 and the disease it causes, COVID-19, has a seasonality, like the flu, that will aid in its disruption this summer.
President Donald Trump has repeatedly stuck to that prediction.
“People are talking about July, August, something like that” when the outbreak “washes though,” Trump said Monday.
John Nicholls, a clinical professor in pathology at the University of Hong Kong, told investment analysts last month, “Three things the virus does not like: 1, sunlight. 2, temperature. And 3, humidity.”
Tropical Hawaii has all three. But it remains unclear whether COVID-19 will follow the mainland pattern of viruses such as influenza spiking in the winter and falling in the summer — and whether that will work in the favor of the Aloha State.
The novel coronavirus appears to be more contagious and has a higher mortality rate and longer incubation period than seasonal flu.
Hawaii doesn’t closely follow mainland patterns for the flu, either, experts note. The CDC on its website addressed the question of whether warm weather will stop the outbreak of the coronavirus.
“It is not known yet whether weather and the temperature impact the spread of COVID-19,” the CDC said. “Some other viruses, like the common cold and flu, spread more during cold weather months, but that does not mean it is impossible to become sick with these viruses during other months.”
The CDC added that “at this time, it is not known whether the spread of COVID-19 will decrease when weather becomes warmer.”
Marc Lipsitch, director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health, wrote that there may be “modest declines” of COVID-19 transmission in warmer weather, but not enough “to make a big dent.”
“Based on the analogy of pandemic flu, we expect that SARS-CoV-2, as a virus new to humans, will face less immunity and thus transmit more readily even outside of the winter season,” he said.
Some factors are known. Viral water droplets from a sneeze, cough or simply exhaling hang longer in the air in cold, dryer climates — adding to infection time — but with higher relative humidity, such as in Hawaii, those droplets gather water and fall quicker.
COVID-19 seems to be transmitted mainly via those respiratory droplets, according to the European Centre for Disease Control and Prevention.
Nicholls said a coronavirus also can survive on a stainless-steel surface for 36 hours. Sunshine, however, works as a sanitizer.
David Cennimo, who studies infectious diseases at Rutgers New Jersey Medical School, told CNN that “many experts hope — and I think the correct word is hope — that the summer will push down the case numbers.”
Part of the theory is that flu is passed more readily on the mainland when folks are cooped up inside in the cold. Schools are an effective transmitter.
Summer not only brings heat, humidity and sun, but also more separation of people.
“These are all things that might help, but really, it’s going to take social distancing” with community spread, said Dr. Melinda Ashton, a pediatrician, chief quality officer at Hawaii Pacific Health and clinical professor with the University of Hawaii School of Medicine.
Ashton said it’s possible that “if we stay 6 feet apart, we would have a lower risk of infection (in Hawaii) than perhaps in a very cold, dry climate — but we mostly don’t stay 6 feet apart.”
The largest clusters of the coronavirus initially were in a colder-weather band that included China, South Korea, Italy and Iran, but COVID-19 has started to spread to warm-weather climes including Thailand and Australia.
A new study on temperature, humidity and latitude analysis to predict the spread and seasonality of COVID-19 notes the disease had established significant community spread in cities and regions along that narrow east-west northerly distribution with average temperatures of 41 to 52 degrees and low humidity — factors consistent with the behavior of a seasonal respiratory virus.
“The number of patients and reported deaths in Southeast Asia is much less” when compared with the more temperate regions noted above, according to the study that included the University of Maryland School of Medicine.
Amesh Adalja, an infectious disease physician at John Hopkins Center for Health Security, told National Public Radio last month that the explosive person-to-person spread in China made the ailment appear to behave like a common cold-causing coronavirus.
“I do think seasonality will play a role,” he said. “As this outbreak unfolds and we approach spring and summer, I do think we will see some tapering off of cases.”
Another hypothesis in the winter-summer virus dropoff is that vitamin D levels boost the immune system as the result of greater ultraviolet light exposure in the summer.
“The best evidence for the relevance of this hypothesis is that vitamin D supplementation reduces the incidence of acute respiratory infection,” Harvard’s Lipsitch said.
Ashton, with Hawaii Pacific Health, said that in Hawaii “we’ve seen flu in pretty much every month of the year.” The state Health Department puts the blame on the high volume of tourism.
There is also seasonality in Hawaii for the flu, but less so than in temperate locales on the mainland, Ashton said.
Ashton said that at some point in the future, “we’ll be able to look back and say, now we know” about COVID-19 and the impacts of weather on it. “But we don’t know now.”