Coronaviruses don’t like the heat. In fact at 38 deg C they have a massive drop in replication and infectivity.  In the lab they are grown at 34 deg C. 
Coronaviruses inhibit the innate immune system: the first line of the body’s defence against viruses. In particular they directly impair the release of the “distress beacon” cells send out to say they are infected (Interferon Type 1 “IFN Type 1”) and (either consequently or directly) inhibit the cell lines (eg monocytes and natural killer cells) in the innate immune system. [3,4] These messages and cells are essential to switch on a normal, healthy, adaptive immune response rather than the late, overactive and damaging immune response seen in hospitalised patients with COVID-19.
Cold temperatures impair the immune system’s ability to release IFN Type 1 when infected with a virus (in mice). 
So being cold both improves the growth of coronaviruses and impairs the body’s distress signal to show it is under attack, which is already being directly inhibited by the coronavirus.
Older people, and those with obesity (with or without the diseases associated with carrying extra weight) have impaired innate and adaptive immune systems, including a decreased ability to mount a fever to an infection. They also have decreased numbers and activity of cells such as natural killer cells and monocyte function [6-8].
So older and unwell/obese people are hit doubly hard by their own weakened innate immune response and the active attack of that area of the immune system by the virus that causes COVID-19.
Fever is a broad based stimulant to the whole of the innate and adaptive immune system . In particular, in respect to COVID-19 infection, fever or hyperthermia increases the release of INF Type 1 and monocyte function [10-12] and directly inhibits viral replication . There are similar results to the immune system if the body temperature is raised by external means eg sitting in a hot bath, as by internal means such as infection .
So inducing mild hyperthermia by water (also known as “hydrotherapy”) could directly
- inhibit replication and activity of coronaviruses
- overcome coronavirus inhibition of IFN Type 1, likely increasing the chances of normal immune resolution of COVID-19
- Enhance immune function in those most vulnerable to immune complications of COVID-19
Hydrotherapy was used widely in Adventist hospitals in the Spanish Influenza pandemic with a mortality rate of 1% compared to 13% in the army-run “open air” hospitals considered state of the art at the time. Average mortality was 40% for hospitalised patients in traditional hospitals [13-15]. (NB these are old figures and not subject to the academic standards of research and trials of today. Nonetheless they are interesting to note.)
Hydrotherapeutic treatments have received little study since the advent of vaccination and effective antibiotic therapy. Consequently there are no randomised controlled trials to support their use in coronaviruses of any sort, let alone COVID-19, but for the above reasons it is plausible to think it should be useful. Early preparations are underway for a trial in Australia. In other cities around the world there is more urgency.
It makes sense to use the time at home after diagnosis of COVID-19 to support the immune system to clear the virus, and increase the chance of a good resolution. It feels like a better alternative than to just cross your fingers and hope you are not one of the 20% who need to go to hospital, 5% who need ICU or 2.5% who do not survive. There are multiple drugs and immunotherapies being trialled but most of these are being focused on the late stages of the illness and are not readily available now. Besides hydrotherapy it makes sense to eat nutritious food, get enough rest, and manage stress. Quitting smoking and limiting your alcohol intake (if you do drink) are also great ideas for this time.
Hydrotherapy is a medical treatment like any other with risks and benefits. Please read the advice about contraindications and safety precautions carefully, and DO NOT USE HYDROTHERAPY INSTEAD OF APPROPRIATE MEDICAL CARE. You should use the treatments outlined on this website only with the support of your local doctor, who can help you determine if it is safe for you, or help you modify them to make them safer for you.
(Written 10/04/20 by Dr Emma Campbell, BMed FRACGP)
(If you have little time, focus on numbers four and nine).
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