The U.S. Centers for Disease Control and Prevention’s provisional death counts for COVID-19 show a striking change. While starting at zero in February 2020 and spiking up to more than 5,000 deaths per week for the oldest age range (85 and over) and 111 among 25- to 34-year-olds in late April, they’ve plummeted.

There were 199 COVID-19 deaths for the week ending May 30, 2020, among those 85 and over, while only one death was reported among 25- to 34-year-olds — an extremely rapid decline from April to May.1 What happened to make the deaths come to a standstill, according to some experts, might be the same seasonal ebb and flow that happens with many respiratory infections: Summer may have killed it.

Many Respiratory Infections Decline in the Summer

While it’s possible to get respiratory infections like influenza any time of year, influenza is more common during the fall and winter, hence the “flu season” during those months. Respiratory syncytial virus (RSV), a leading cause of severe respiratory illness in young children and those aged 65 and over, is also more common in the fall and winter.

At least four common coronaviruses are also highly seasonal with transmission similar to influenza.2 Although these aren’t related to SARS-CoV-2, the virus that causes COVID-19, it is genetically related to the coronavirus responsible for the severe acute respiratory syndrome (SARS) outbreak of 2003.

This is notable because, as noted by professor Paul Hunter from the University of East Anglia in England, “Sars largely spread in hospitals but still died out in the summer in the Northern Hemisphere.”3 There are a number of reasons why SARS was quickly contained in about eight months, but the summer, with its higher temperature and humidity level, is among them.

What Makes Summer Less Conducive to Respiratory Infection?

Winter’s dry, cold air is favorable to the spread of flu transmission, and influenza spread is known to be affected by both temperature and humidity.4 During the winter, people also spend more time indoors, in enclosed spaces with less ventilation and less personal space compared to being outdoors in the summer.5

School is usually in session during the fall and winter, with students at home over the summer. School terms have been associated with higher transmission of respiratory viruses, while holidays lead to a 20% to 29% reduction in the rate at which influenza is transmitted in children.6 So, just the fact that children are in school in the winter may raise transmission rates.

What’s more, as noted by Marc Lipsitch, professor of epidemiology and director of the Center for Communicable Disease Dynamics at the Harvard T.H. Chan School of Public Health:7

“It is possible that the condition of the average person’s immune system is systematically worse in winter than summer. One hypothesis has focused on melatonin which has some immune effects and is modulated by the photoperiod, which varies seasonally. Another with more evidence is that vitamin D levels, which depend in part on ultraviolet light exposure (higher in summer) modulate our immune system in a positive way.”

Studies Suggest COVID-19 May Be Seasonal

A number of studies have suggested COVID-19 may, in fact, taper off during the summer. One preprint study tracked the seasonality of influenza viruses and endemic human coronaviruses over an eight-year period. The activity of human coronaviruses peaked the first week of January, with transmission facilitated by low indoor relative humidity (RH) of 20% to 30%.8

The researchers cited previous studies that found an increase in relative humidity to 50% reduced the transmission of both influenza and animal coronaviruses. What’s more, the study found a decrease in disease incidence by 50% in early March, 75% in early April and greater than 99% at the end of April. According to the study:9

“As a lipid-bound, enveloped virus with similar size characteristics to endemic human coronaviruses, SARS-CoV-2 should be subject to the same dynamics of reduced viability and transmission with increased humidity. In addition to the major role of social distancing, the transition from lower to higher indoor RH with increasing outdoor temperatures could have an additive effect on the decrease in SARS-CoV-2 cases in May.

Over the 8-year period of this study, human coronavirus activity was either zero or >99% reduction in the months of June through September, and the implication would be that SARS-Cov-2 may follow a similar pattern.”

Humidity May Be a Major Factor

A study conducted in Sydney, Australia, found a similar connection between humidity and COVID-19. A 1% decrease in humidity was predicted to increase the number of cases by 6.11%,10 with researchers stating, “During periods of low relative humidity, the public health system should anticipate an increased number of COVID‐19 cases.”11

Professor Michael Ward, an epidemiologist in the Sydney School of Veterinary Science at the University of Sydney, said in a news release that humidity appeared to be a major factor:12

“When it comes to climate, we found that lower humidity is the main driver here, rather than colder temperatures. It means we may see an increased risk in winter here, when we have a drop in humidity. But in the northern hemisphere, in areas with lower humidity or during periods when humidity drops, there might be a risk even during the summer months.

… When the humidity is lower, the air is drier and it makes the aerosols smaller. When you sneeze and cough those smaller infectious aerosols can stay suspended in the air for longer. That increases the exposure for other people. When the air is humid and the aerosols are larger and heavier, they fall and hit surfaces quicker.”

Aside from affecting transmission rates, humidity may also affect the survival of viruses. The addition of a portable humidifier with an output of 0.16 kilograms of water per hour in the bedroom increased absolute humidity 11% and relative humidity 19% during sleeping hours compared to having no humidifier present, according to one study. Along with the increases in humidity came a decrease in the survival of influenza virus, by 17.5% to 31.6%.13

Humidity even influences innate immune defenses against viral infections. In an animal study, dry air compromised the mice’s resistance to infection, and those housed at lower humidity levels had impaired mucociliary clearance, innate antiviral defense and tissue repair function, the study found.14

The Vitamin D Connection

The other reason why summer may slash COVID-19 deaths is because summer equals greater exposure to sunlight, which boosts vitamin D levels. There is strong scientific evidence vitamin D plays a central role in your immune response and your ability to fight infections. It’s been shown in an analysis of 212 people with lab-confirmed COVID-19 that disease severity is associated with vitamin D levels, with lower levels linked to more severe disease.15

A review published in the journal Nutrients also concluded that not only could vitamin D be useful to reduce the risk of infection with COVID-19, but also could be helpful for treatment:16

“Evidence supporting the role of vitamin D in reducing risk of COVID-19 includes that the outbreak occurred in winter, a time when 25-hydroxyvitamin D (25(OH)D) [vitamin D] concentrations are lowest; that the number of cases in the Southern Hemisphere near the end of summer are low; that vitamin D deficiency has been found to contribute to acute respiratory distress syndrome; and that case-fatality rates increase with age and with chronic disease comorbidity, both of which are associated with lower 25(OH)D concentration.”

If COVID-19 is seasonal, a resurgence is likely come fall, which is why the time for optimizing your vitamin D level is now. To improve your immune function and lower your risk of viral infections, you’ll want to raise your vitamin D to a level between 60 nanograms per milliliter (ng/mL) and 80 ng/mL by fall. In Europe, the measurements you’re looking for are 150 nanomoles per liter (nmol/L) and 200 nmol/L.

Why Do so Many Experts Get It Wrong?

Harvard professor Lipsitch is among those who said COVID-19 would “probably not” go away on its own in warmer weather. “The short answer is that while we may expect modest declines in the contagiousness of SARS-CoV-2 in warmer, wetter weather and perhaps with the closing of schools in temperate regions of the Northern Hemisphere, it is not reasonable to expect these declines alone to slow transmission enough to make a big dent,” he said.17

CDC’s provisional death counts appear to suggest otherwise, but some have cautioned that COVID-19 is too new to be seasonal. In other words, because fewer people have established immunity, a new virus has an advantage in that it can thrive even in less-than-optimal conditions for a virus, i.e., the summer.

“Old viruses,” Lipsitch said, “which have been in the population for longer, operate on a thinner margin — most individuals are immune, and they have to make do with transmitting among the few who aren’t.”18

Likewise, a study in Science used a computer model to suggest that while COVID-19 may fall into seasonal patterns eventually, this may not occur until more people develop immunity, noting that “susceptible supply” is limiting the role of climate in the early COVID-19 pandemic.19

The drastic decline in COVID-19 deaths that occurred from April to May do suggest a seasonal component, but what’s driving the drop is not completely understood. It’s likely a combination of humidity, heat, human behaviors, vitamin D levels and, likely, other aspects of sunlight exposure that are culminating in this decline.

With summer upon us in the U.S., you can use it to your advantage to spend time outdoors, optimize your vitamin D levels and get sensible sun exposure, all of which can help you support health and reduce your susceptibility to viral infections.

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