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Will the Coming Warm Weather Reduce the Spread of COVID-19?

Analysis by Dr. Joseph Mercola Fact Checked

will warm weather slow covid 19

Story at-a-glance -

  • Scientists are anticipating that the lipid layer around COVID-19 will make it sensitive to heat and trigger a seasonal fluctuation, similar to flu
  • Yet, scientists also know pandemics do not follow seasonal patterns; until the virus becomes endemic, it may continue to spread
  • Data show women have a higher survival rate than men; this is also true of the influenza virus, SARS and MERS
  • Vitamin D deficiency, more common in the winter months, contributes to seasonal fluctuations with flu; the only way to know if you're deficient, and may benefit from supplementation, is to test

For everything there is a season, and that includes viruses. Whether it's the common cold or flu viruses, in the U.S. the season begins in fall. By December, the viral spread is in full swing until it peaks in February. However, it’s different in the southern hemisphere where Africa, Australia and Brazil are approaching winter as the U.S. and most of Europe are sliding into spring and summer.

For an understanding of the novel coronavirus (COVID-19) currently spreading across the world, it helps to have some background on the virus that triggers flu. Interestingly influenza viruses1 and COVID-192 have similar characteristics, including a heat-sensitive lipid layer.

The name "influenza" originates from the initial Italian name, which was "influenza di freddo" or "influence of the cold."3 Yet, while the colder months have higher rates of infection, you can still get a cold or flu infection throughout the year.

Other respiratory viruses circulate during the flu season and may trigger similar symptoms. Respiratory syncytial virus (RSV) is one that causes severe illness in young children and, according to the Centers for Disease Control and Prevention, is "a leading cause of death from respiratory illness in those aged 65 years and older."4

Although flu is common and well-studied, it continues to be deadly. While estimates vary from year to year, the CDC estimated that for the 2018-2019 season there were 35.5 million sick with flu, 490,000 hospitalized with it and 34,200 who died because of it.5 But, the reality is these are only estimates since6 "flu deaths in children are reported to CDC; flu deaths in adults are not nationally notifiable."

Rising Flu Infections Compound COVID-19

In order to improve the accuracy of tracking illness and death, researchers look at the rate of death from pneumonia and influenza as most fatalities from influenza involve pneumonia.7 In the 2018-2019 flu season, the CDC found the percentage of death attributed to pneumonia and influenza was at or above epidemic thresholds for 10 weeks.8

The rates in the 2017-2018 season were recorded over epidemic numbers for 16 weeks and exceeded 10% for four weeks.9 While these are concerning, it’s important to know they are based on those who have symptoms or see their physician. Like COVID-19, there are many who carry influenza without symptoms but can infect those around them.

Harvard Health Publishing10 reports that from 20% to 30% may have the virus without symptoms, but data from a community-based study in England11 suggests “most influenza infections are asymptomatic.” This is similar to what researchers are finding out about COVID-19.

Nearly 80% of people with the novel coronavirus are either asymptomatic or have mild symptoms that don’t present the need for a physician appointment.12 Of the remaining 20%, the World Health Organization13 finds that 13.8% develop severe disease, which they define as significant respiratory distress with blood saturation levels less than or equal to 93%.

Unlike the death percentage from flu that may reach as much as 10% on peak weeks, numbers for COVID-19 indicate the worldwide “shelter in place” has kept the global percentage hovering from 4% to 5%. Of course, this is a global average as there are countries with higher and lower percentages. You can track the changing numbers on the Johns Hopkins map.14

Remember, the reported numbers of people who have been tested for COVID-19 are used in the map and not the total number who may or may not have symptoms. While just one death is too many, social distancing, frequent hand-washing and an understanding of how the virus spreads may keep the death rate lower than that from pneumonia and influenza.

The strain on medical resources commonly experienced during flu season is currently compounded by COVID-19, which also causes respiratory problems. These are important facts to remember as you read the headlines and emerging news reports.

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COVID-19 May Recede in Summer Months

Just as flu has a season based on environmental temperatures, scientists are anticipating the same for COVID-19.15 Researchers studied the infectivity from January 21 to January 23, before the Chinese government put measures in place January 24 to stop the spread of the virus.16 This gave the scientists data on its natural, unhindered viral spread.

They found in the early stage of the outbreak, areas with low temperatures and lower humidity had higher numbers infected than those with warmer and more humid climates. Data from 14 countries with more than 20 cases from February 8 to February 29 revealed that its severity was dependent on temperature and humidity. The higher both measurements, the lower the severity during that time.17

Whether warm weather will help to flatten the curve and thus reduce the strain on medical resources remains a question.

Typically, this type of virus does exhibit seasonality, yet, as the BBC reports, pandemics may not follow seasonal patterns. Jan Albert, professor of infectious disease control at Karolinska Institute in Stockholm, addressed the question of whether there would be a seasonal flow:18

“Eventually we would expect to see Covid-19 becoming endemic. And it would be really surprising if it didn't show seasonality then. The big question is whether the sensitivity of this virus to [the seasons] will influence its capacity to spread in a pandemic situation. We don’t know for sure, but it should be in the back of our heads that it is possible.”

Endemic infections are those that have been moving through the human population for years. Many of the coronaviruses have been endemic and therefore have responded to seasonal changes,19 giving researchers hope that COVID-19 will respond similarly as the spring and summer months arrive.

Experts across the world have tried to estimate the spread of the disease and believe it hovers between 2.0 and 2.5, which means for every infected person there will be two to 2.5 who become infected.20

One reason researchers expect a summer drop in transmissibility is humidity. Virologist Thomas Pietschmann from the Center for Experimental and Clinical Infection Research in Hanover, Germany, explains:21

"What's special about this virus is that humans are confronted with it for the first time. From the data we have from China, we can conclude that the virus has only once passed from an animal to humans and spread from there.

Viruses have greater stability at low temperatures. This is similar to food that keeps longest in the refrigerator. On cold and usually dry winter days, the small droplets, together with the viruses, float in the air longer than when the air humidity is high."

Viruses Affect Gender Differently

Data have also shown that women have a higher rate of COVID-19 survival than men;22 this is similar to the flu.23 While it appears men and women have been infected at the same rate, the percentage who have died is greater for men. The New York Times reports the Chinese CDC published an analysis showing COVID-19 deaths in men numbered 2.8%, but in women it was 1.7%.

Researchers found the same thing with the SARS and MERS virus outbreaks. Some experts believe the differences may lie in the ability to mount a better immune response against infections.

Seasonal Fluctuations May Be Related to Vitamin D

While many believe getting the flu shot will reduce your risk, the effectiveness is usually less than 50%.24 Instead, you may have a better chance of reducing severity and infectivity by paying attention to your vitamin D levels. Many believe the rise and fall of infections are based on three factors, two of which involve your immune system:25

  • Spending more time indoors during the winter months with the windows sealed means you’re more likely to be infected breathing the same air as someone who is carrying a virus.
  • The days are shorter, and the lack of sunshine affects your vitamin D and melatonin levels, which in turn impacts your immune system.
  • The virus survives better in cold dry climates.

As far back as 1981 researchers were proposing there was a seasonal reason behind influenza epidemics. In a review of published literature,26 researchers noted that the robust seasonal vitamin D production, normally an option during the summer months, leaves people deficient in the winter.

This deficiency predisposes children to respiratory infections. They also found adults injected with live influenza virus were more likely to develop symptoms when their vitamin D level was low. They concluded that the seasonal stimulus proposed in 1981 was a function of vitamin D deficiency.

Some scientists have suggested that vitamin D induces another molecule, cathelicidin, to activate genes involved in the immune system.27 GrassrootsHealth reports that several randomized controlled trials have demonstrated the effectiveness vitamin D has in reducing the risk of contracting the flu, regardless of the mechanism.28

To fully support your body's ability to fight infections, you're aiming for a level between 60 and 80 ng/mL, with 40 ng/mL being the lowest cutoff for sufficiency. Although research may suggest how much it would take for the average person to achieve 40 ng/mL, your individual requirement may vary.

To know if you need a supplement or not, you'll need to get your levels tested. Regular exposure to sunlight is the ideal way to optimize your vitamin D levels, but this may not be possible during the winter months if you live in the northern hemisphere.

GrassrootsHealth makes testing easy by offering an inexpensive vitamin D testing kit as part of its consumer-sponsored research. By signing up, you are helping further vital health research that can help millions in coming years. All revenues from these kits go directly to GrassrootsHealth. I make no profit from these kits and only provide them as a service of convenience to my readers.

Support Your Immune System and Reduce Infections

There are several strategies you can use to support your immune system and reduce the number and severity of infections. Your immune system is your first line of defense against infection and to support health.

In addition to optimizing vitamin D and the suggestions listed below, you’ll also want to wash your hands frequently to reduce the spread of viruses and bacteria that trigger infections. For information on effectiveness and technique see “The Impact of Effective Handwashing Against Infection.”

Nutrition — The role of your gut microbiome in your immune system function has been well documented in the past years.29,30,31 To support your gut microbiota it's important to feed and care for your beneficial bacteria.

You can do this by limiting antibiotics to critical use, eating a high fiber diet to feed your microbiota, and including fermented foods in your nutritional plan to populate your gut with beneficial bacteria. Steer clear of sugar and processed foods, which feed harmful bacteria.

Sleep — There is a high cost to sleep deprivation, including damage to your immune system. As I've written before, a lack of sleep increases your risk of obesity, dementia, cancer, osteoporosis and results in premature aging. If you have trouble sleeping, see "Top 33 Tips to Optimize Your Sleep Routine" for suggestions to help you achieve quality sleep.

Exercise — Regular exercise can reduce the potential you'll get infected with a virus, with at least one study reporting the reduction being32 as high as 46%. Even those who were infected experienced a reduction in the severity of their symptoms.

Since exercise has repeatedly been proven to boost your immune system, treat it like a crucial part of your daily routine. However, not everyone relishes the idea. If you need help adding exercise to your routine see "Here's How to Make Yourself Love Exercise."