This week we devoted our program to weather, and at the time we scheduled it, it seemed like a good fit for mid-March. Events have a way of overtaking our plans, but a question I have been tracking might make this topic a little more relevant in the current circumstances: how does weather affect the spread and virulence of disease, and especially COVID19?
It has long been known there is a flu season, and it begins each fall and lasts too long, but cases taper off in the spring. There are three popular theories why this happens.
- We spend more time inside, closer to other people during cold weather and transmission of the virus is easier. Indoor air in the winter is much drier than in the summer, which can make our nasal passages less effective at trapping pathogens. This is almost certainly a factor.
- Sunlight hours decrease in winter, lowering our production of Vitamin D and diminishing our immune system. This one doesn’t have strong evidence, but it cannot be ruled out.
- Flu viruses prefer cool dry air to thrive in. This reason is almost certainly a powerful factor. Apparently in higher humidity the airborne virus absorbs the available water vapor, becomes too heavy to stay airborne, and falls to surfaces. Relative humidity can plummet during winter, so the investment in a humidifier could be a wise decision.
This is one reason many of us get colds after long plane flights – the air on high-flying jets is drier than a desert and being crammed into an aluminum cylinder with a few hundred strangers makes a perfect combination for aerosol infection. The only problem with this temperature/humidity link is it seems to be true only in locations that have cool, dry winters. Flu transmission rates in warm climates indicate the virus can thrive in high humidity as well.
What does this mean for the coronavirus? We really don’t know, but right now this chart doesn’t show much difference in transmission rates between warm locations and cold locations. So will warm weather help lower incidence? Probably, but the bigger point is that doesn’t mean it goes away. After all, regular flu virus comes back every year by surviving during high heat and humidity somehow. Recurrence this fall is highly probable. COVID19 is so easily spread between people that the southern hemisphere will have an infected population that can travel to re-infect the north after our summer is over. We need a vaccine, not just a change of seasons.