So much has been surmised and written about the novel Coronavirus (COVID-19) along with the will reach a peak and then decline. Let’s rephrase this. COVID-19 will not go away. What will happen is as the pandemic slows, the restrictions in place for social distancing will be relaxed. That inevitably will result in a second rise in the number of cases of COVID-19.
Current estimates are that this may occur in late summer. We are, however, hopeful that when the second wave arrives, it will be less severe than the initial pandemic. Many people will be immune and hopefully testing will be more available so that infected people can be quarantined promptly.
Some people have suggested that COVID-19 may not be as infectious in summer akin to other viruses that are seasonal. That raises two questions: are viruses really seasonal and could COVID-19 be one of the seasonal viruses? Since COVID-19 has just recently been discovered, it is not possible to determine if it really is seasonal or not, but let’s look at history for some clues.
Seasons in the Sun?
So, what’s the story about seasonality and viruses? Common folklore has it that the flu is seasonal. Both Hippocrates and Thucydides identified that some diseases were seasonal. But in spite of this being recognized for millennia, there is actually very little research on the topic.
The flu does occur in the winter months. So, if COVID-19 acted like the flu, then when the second wave hits, there would be less spread because COVID-19 would less infectious in late summer. But some viruses have different peaks with some peaking in almost every quarter of the calendar year, and some varying, depending upon the latitude. For example, polio was most active in the summer months which led to the closing of swimming pools.
A publication by Martinez (2018) identified 68 viruses that were seasonal, but they were not synchronized as to when they occurred. There are viruses that demonstrate seasonality but any month of the year can have a seasonal virus in it. This pattern argues against changes in human behavior as being the reason for the seasonal changes. So, for example, children returning to school in the fall would not explain why some viruses are more active in the summer when kids are out of school with less contact to one another.
It’s what inside that counts
One virologist has proposed that it is the behavior of the virus outside of the body which helps to explain seasonality. One possible explanation relates to the fact that some viruses are packaged with just a protein shell while others have a membrane (envelope) made of lipids covering them. COVID-19 does have an envelope. [See aside at end of article] A virus with an envelope is more fragile and thus more susceptible to summer heat. In 2018, Dr. Ramalingan published a study attempting to help understand the seasonality of some viruses. He studied nine viruses in 36,000 respiratory samples taken over a six-year period of time. Some of the viruses had envelopes and others did not. He found that viruses with an envelope exhibited seasonality. He found that RSV, human metapneumovirus and the flu all had an envelope, and their incidence peaked in the winter months.
However, and somewhat surprisingly, rhinovirus (cause of the common cold) was not seasonal and lacks an envelope. The study found that the samples were positive on 84.7% of the days of the year but did increase when children return to school. So, children returning to school can cause a rise in the common cold, but the cold can occur at any time during the year. The study also found that the frequency of influenza and RSV were lowest when the relative humidity was lower than the average relative humidity. Another research has proposed that it is the absolute humidity, the total amount of water in the atmosphere, that matters. Why changes in humidity would reduce the frequency of viruses that have an envelope is unknown. Four other coronaviruses that cause colds and have an envelope do have seasonal variability with the peak incidence in the winter.
What does it all mean?
So, will COVID-19 exhibit the seasonality of the flu and thus the second wave will be less than it might have been due to the occurrence of the wave during the hot months of the summer? Two conflicting reports have been published based upon the experience in China. One report states that the frequency of the virus does not vary with the temperature of the province. A second report says that the virus only occurs in specific temperature bands with a humidity greater than 70%. Another possibility is that human immunity might change with the season, a phenomenon that occurs in some animals. A study evaluating the human immune genes found that 4000 genes do exhibit seasonality.
The current novelty of COVID- 19 precludes knowing if the second wave of infection will be reduced by the virus’s seasonality. That certainly would be nice but so far COVID-19 has been anything but nice.[Aside: The amazing capability of understanding how biology works find no better example than in a study being done at University of Texas at Austin’s Texas Advanced Computing Center (TACC). The researchers are constructing an all-ATOM molecular model of COVID-19’s envelope using a supercomputer. The skeptics may say “so what” but having this detailed model will advance the understanding of how COVID-19 recognizes angiotensin-converting enzyme 2 (ACE-2). The virus enters a cell by interacting with ACE-2 so knowing explicitly how this interaction occurs can promote the development of drugs that will alter or block this interaction. The researchers estimate there will be over 200 MILLION atoms- just a mind-boggling accomplishment.]