The influence of humidity on the spread of the COVID-19 virus

Non (sub-)tropic regions usually experience flu epidemics in winter. This leaves us with an interesting question: Will the COVID-19 virus follow this seasonal course?

There are strong indicators that this will indeed be the case. However, in the meantime there have been more clues that strengthen this hypothesis.

Below you will find a short summary of these clues. I have attached links to the sources I used so you can read the supporting research for yourself.

What is the deciding factor in the seasonality effect of influenza?

The most important (and biggest) research report I could find is an article published in 2013 in The Public Library of Science.

The article covers a meta-study involving 78 different places, where for each it has been tracked for at least two years where an INFLUENZA epidemic started and what the weather was like at that time. From this data it has been concluded that the so called ‘specific humidity’ is a very good indicator for predicting the moment of outbreak in non (sub-)tropical regions.

Look at the graph below. The vertical columns indicate the average specific humidity per month. (Which goes from less than 1 gram of water per kilogram air up to almost 18 grams water per kilogram air.)

Each dot represents a place where the following month a flu epidemic occurred. Out of the 78 testes, 44 places with an influenza outbreak were located above 25 degrees latitude (areas with heavy winters) i.e. when the specific humidity was at its lowest. More importantly, there seems to be an upper limit of around 6 g/kg.

The literature gives two different possible explanations for the relation between humidity and the outbreak of flu epidemics, being:

  • The drops left when an infected individual sneezes or coughs, stay afloat much longer in a dry air than in a ‘wet’ or humid air, according to Dr. A. Evangalista.
  • The influenza-virus has -just like COVID-19- a so called lipid envelope. Scientists argue that this envelope, or membrane, is damaged by the water in the air, thus dismantling the virus. This happens significantly less in regions with low humidity, as I read here.

So, the relation between the influenza-virus and the specific humidity has been proven. But can we say the same for the COVID-19 virus?

The good news is that the analyses of the virus’ global spread (as described both by me and by scientific literature) indicate a similar connection. Plus, the 6 g/kg limit of specific humidity seems to play an important role here too.

An important research paper published in early March looks at the COVID-19 outbreaks worldwide. It concludes that each outbreak occurred right after a period of temperatures between 4 and 11 degrees Celsius and a specific humidity below the limit of 6 g/kg. All major outbreaks worldwide are in the same “bandwidth” across the globe.

My own analyses of the global COVID-19 course show similar patterns, especially for Italy, the Netherlands and the USA.

The graphs below, for example, show the humidity in four different Italian and Dutch cities in February and March this year. Written behind the city’s name is the number of days the humidity was below 6 for an average humidity measured at 2 pm. The graph underneath the four graphs shows the relations between the average humidity and the number of deaths in those places.

This graph shows the development of the death rate in the four regions, using the number of days after the tenth reported infection as criterion.

It’s clear that the death toll in the provinces of Rome and Naples are significantly lower than those from Bergamo and Milan. The Netherlands are somewhere between Lombardy (Bergamo), Naples and Rome. Italy locked down at the 9th of March, so we should start to see the effect of that right about now for Naples and Rome.

All these numbers support the findings of a clear relation between the intensity of the outbreak and the specific humidity and it is very likely that the previously mentioned 6 g/kg is an important threshold value.

The USA’s outbreak pattern is similar: when population size is accounted for, there are much less deaths in Florida and California than in New York and Washington State (Seattle). Here too we can see that with regards to the specific humidity there are similar trends to other investigated places. Seattle and New York both had way more days below 6 g/kg than Miami or Los Angeles.

There are two interesting states that give a remarkable (and valuable) number. Illinois (home to Chicago) has, so far, few deaths. This follows the hypotheses that when the temperature is really low (regularly below freezing) circumstances are unfavorable for spreading the virus.

In Louisiana, where the clear outbreak assigned to Mardi Gras (New Orleans’ carnival festival) on the 25th of February, the specific humidity was way above 6 g/kg (even more than 15 g/kg). So why the outbreak and death toll?

Something similar happened in Malaysia. In the last four days of February a big religious Islamic convention was held, involving around 16.000 participants, 1.500 of which were foreign. Malaysia also has a high specific humidity.

I will soon write a separate blog on how that is possible and why the Malaysia outbreak led to far less deaths than the Louisiana one.

Finally, there has been a Chinese research report, which also draws a line between the weather and the so called R0-value (how many people one individual infects) based on their findings in Chinese cities. As the temperature and/or humidity rises, the R0, according to this report, decreases.

All these results make a strong case for the hypothesis that specific humidity influences how fast the COVID-19 virus spread.

The fact that there are way fewer deaths in Rome and Naples than in Bergamo is mostly due to that very relation! Had the Dutch death toll followed a similar trend then we would have between 100 and 200 deaths instead of the now (estimated) 450.

This science can be of use in forming the policy with regards to lowering the R0 (speed of infection) and the exit-strategy (how we can slowly return to a normal situation). I will go into this in a following post.

These 6 blogs form a coherent whole:

How great is the danger of the COVID-19 virus really?
The influence of humidity on the spread of the COVID-19 virus (This blog)
What is happening in the (sub)tropical areas with the spread of COVID-19?
How do we respond to the positive effect of a higher specific humidity?
What if the diffusion rate of COVID-19 has dropped to -near-zero?
The enormous economic and social consequences of this pandemic


What will happen to the spread rate of COVID-19 in (sub-)tropical areas?

Research from 2013 concerning the seasonal sensitivity of the influenza virus saw a marked shift in patterns between areas above the 25th Latitude toward the Pole (including the southern hemisphere), and the areas between the equator and the 10th Latitude.

In the areas above the 25th Latitude a clear pattern arises, showing peak influenza numbers in winter (explained by low air humidity). In areas close to equator a peak is seen when air humidity is at its highest and when it rains the most (such as during Monsoon season).

As of yet, there is no explanation why influenza thrives most in areas with high temperature (close to 37 degrees) when there is high humidity, while the virus seems to thrive most in lower temperature areas when there is low air humidity. It could be that hot conditions with high rainfall drive people into their homes and closer together, allowing for a higher rate of infection.

Whatever the case may be, the question is whether the COVID-19 virus will also show similar behavior in tropical areas. The first signs pointing to this being the case deserve closer scrutiny.

In Ecuador (close to the equator) we currently see a death toll of 34. But Malaysia may be the most interesting country to follow. On one hand, Malaysia implemented restrictive measures in an early phase of the outbreak. On the other hand, for the past two weeks Malaysia is seeing a substantial outbreak.

This can be explained in large part by an islamic religious gathering occurring in the Seri Petaling Mosque between February 27th and March 3rd. Around 16.000 people gathered closely together in a religious festival for several days. 1.500 of these people where foreigners, hailing from over 25 different countries.

The Star newspaper is a great source to follow developments in this country, because Malaysia is performing a high number of tests and is also diligently tracking the contacts of persons confirmed to have the virus.

Currently, about 10.000 visitors to the religious festival have been tested and it looks like 15 to 20 percent of these were infected in the 6 days of the festival. Now, we can’t be completely sure whether every attendee was in actual contact with the infected, so these percentages at most show the minimum number of infections.

There is also another number that stands out. As of this moment, only 13 deaths have been linked to people infected during this religious gathering. This is in spite of the fact that we already know that it’s likely a lot more people have been infected than the initial 1,200 that received positive test results. It seems that the mortality rate for those infected there will not reach 1%.

As of yet, we don’t know the age composition of the attendees of the event. It’s difficult to deduce this from photographs.

After the festival, infected worshippers have continued to infect others, while the current death toll in Malaysia amounts to (only) 26.

Mardi Gras, held in Louisiana on the 25th of November, also seems to have been a clear catalyst for spread of the virus. Large crowds of people in close proximity truly provide the ideal circumstances for a fast and widespread transmission of the virus. It’s not clear however, why such a fast transmission of the virus, similar to the one in Lombardy, can occur in areas with high specific humidity, like Louisiana.

Today, for example, the temperature reached 29 degrees with 60% air humidity in New Orleans. This is similar to current weather in Malaysia (32 degrees and 65% air humidty). It is clear that in Louisiana the outbreak is quicker than in Malaysia, probably because in the latter country they tried to find the origin on a case-by-case basis and then took drastic quarantine measures.

It’s still to early to draw conclusions from this, but it’s my impression that COVID-19 does follow the pattern of influenza virus in tropical areas. This would mean that the transmission rate should be higher in the tropics than in areas with high air humidity at more northern latitudes. I estimate that R0 (the amount of other people infected by 1 infected person) will be lower in areas that show a combination of high air humidity and low temperatures than areas that show a temperature between 4 and 11 degrees and low air humidity. Although further developments in Louisiana might disprove this.

Perhaps (and hopefully) we will see an R0 that will be lower than that of influenza virus. Only time will tell. This is especially true, because we aren’t yet in the phase where rainfall is at it’s highest in these tropical areas. In India, the monsoon season usually starts in April. If R0 is truly higher during these humid times, then India will be facing a difficult situation.

Looking at India, it’s also interesting to note whether there will be any differences in transmission rates when comparing the north-south divide. After all, India is a vast country that stretches from the 30th Latitude all the way to the 10th .

These 6 blogs form a coherent whole:

How great is the danger of the COVID-19 virus really?
The influence of humidity on the spread of the COVID-19 virus
What is happening in the (sub)tropical areas with the spread of COVID-19? (This blog)
How do we respond to the positive effect of a higher specific humidity?
What if the diffusion rate of COVID-19 has dropped to -near-zero?
The enormous economic and social consequences of this pandemic


What if the spread rate of COVID-19 almost reaches 0?

It’s currently estimated that, without any special measures taken to reduce the spread, the COVID-virus has an R0 of 2,2. This means that 1 infected person will infect 2.2 others.

Needless to say, if you make sure that an infected person has reduced or no contact with other people during his infection, the R0 will decrease, possibly all the way down to 0.

But in order to achieve a perfect quarantine, the infected person must know of his or her infection at the earliest stage of the disease.

For the time being, the best policy to reduce R0, is to make sure that infected persons have as little social contact as possible. Social contact should especially be avoided with people who have a higher risk of death when contracting the disease. A wide range of measures is being implemented worldwide to achieve this.

This raises some questions: To what extent do these measures work in practice, and how long can these measures be sustained before the cure becomes worse than disease?

It’s clear that a complete lock down, as well as the lighter form practiced in Holland, reduces R0 substantially. Currently, it is estimated that the R0 in the Netherlands is reaching 1,0.

China and South-Korea have managed to reduce their R0 to a value close to 0. But it is questionable whether it is possible to apply this strict approach in Holland, given the differences in culture with these Asian countries.

Finally, the key question will be at what point the social distancing measures can be (slowly) reduced. This is called the exit-strategy. What risks remain and will we be willing to take them? Which activities can be permitted again, and which do we forbid? And finally, given the possible health risks and the declined state of our economic and social infrastructure, what activities are we still able to do? Thus far, little is said about the latter. But it takes little imagination to know that there is still a lot to be said and done about this.

I will devote several blogs to this in the future.

These 6 blogs form a coherent whole:

How great is the danger of the COVID-19 virus really?
The influence of humidity on the spread of the COVID-19 virus
What is happening in the (sub)tropical areas with the spread of COVID-19?
How do we respond to the positive effect of a higher specific humidity?
What if the diffusion rate of COVID-19 has dropped to -near-zero? (This blog)
The enormous economic and social consequences of this pandemic


How do we handle the positive effect of a higher specific humidity?

The following question must be faced quicker than many people think: How long will we keep up the hard measures aimed at decreasing the chance of infection? Meaning, bringing the R0 (the number of people infected by one single individual) back to (almost) 0. Lees meer

The Enormous economic and social consequences of this pandemic

In 1995 I wrote a book about my expectations of a developing revolution: the internet. This article was written about it by a journalist, 21 years later.  Many of the developments I anticipated became fact.

What was my secret?

I noticed that others were mainly focused on a single predictor when making anticipating the future, rather than a combination of factors that also influence each other.

Yesterday I saw the same mistake being made again by the Dutch Central Planning Bureau (CPB) presenting their expectations of social and economic developments resulting from the Corona crisis. We have a tendency to look at isolated factors rather than seeing the interactions between the factors.

It is a complete illusion to think that economic and social life would revert back to normal when the Netherlands reverses the measures taken the last couple of days.

Firstly, social behaviour will be different. Many will behave in a more reserved way when meeting others than they would have before the crisis. When can we feel safe again to go to a restaurant, the cinema, a busy shopping centre, the market or on holiday?

Secondly, many companies responsible for no small part of current employment will have disappeared. And even if the situation in the Netherlands would normalize again, this is no guarantee that the same is true for other nations. This will have grave consequences for exports and imports.

Thankfully the coffers of the Dutch state go deep enough to provide support for most of those who will get hit by the crisis. But, again, this a position of privilege compared to those of other countries.

All things considered, we should prepare for a social and economic earthquake of massive proportions. The consequences will be far more dire than we currently anticipate.

However, this instills less fear in me than another development reflected in history.

Up until now, most citizens understand the measures taken by their governments because they realize the importance of them for their own health and that of their loved ones. But at the same time, all around the world, these far-reaching measures are creating problems. People are losing their jobs. Some, already stricken by poverty, will face even more pressure than they were already facing. Economic inequality will sooner rise than fall.

It’s not hard to imagine that a previous fear of hunger or illness will be amplified by COVID-19. This could lead to people rising up in different ways. Such as protests, civil disobedience, looting etc.

The government will no doubt try to suppress these forms of protest which risks further escalation. These could lead to more than just local conflicts. This presents a genuine and severe risk.

Of course, I don’t hope this will happen. But I fear that, as the current situation persists, these risks will keep growing. International tension might rise to an uncomfortable level during this process.

If this happens, we truly have reached scenarios previously only restricted to movies. There usually is a hero or event that delivers a happy ending in these stories, but I’m afraid that this is the only part  that will remain fictional.

All things considered, I must conclude that both the severe, early measures designed to contain the virus as well as premature measures taken to reduce the pain and start recovery may lead to severe negative consequences.

In this process, we can be sure that the necessary popular support and unity necessary for far-reaching measures will not always exist, further inflaming tensions.

I hope, not only our policymakers, but for ordinary citizens as well, that they can find strength and wisdom in these trying times.

In this blog I will keep sharing my thoughts and findings. I hope this will help sharpen the mind of others who are thinking about this and trying to find their responsibility.

Try following the links that I present. But make sure to also pay attention to the arguments and perspectives of those who disagree with me.

Last but not least: I once more urge our government to do more research into the relationship of high air humidity and the speed of transmission of the COVID-19 virus. An investment in this research might be one of the best the government could make right now.

And make sure to involve ordinary citizens in the deliberations concerning our exit strategy. Sooner or later, this will need to happen.

These 6 blogs form a coherent whole:

How great is the danger of the COVID-19 virus really?
The influence of humidity on the spread of the COVID-19 virus
What is happening in the (sub)tropical areas with the spread of COVID-19?
How do we respond to the positive effect of a higher specific humidity?
What if the diffusion rate of COVID-19 has dropped to -near-zero?
The enormous economic and social consequences of this pandemic (This blog)

Lockdown light: smarter and better

This is the article:  Lockdown Light: smarter and better
Here you will find  the calculator for the “specific humidity”.