I have been working in India and Singapore for the past two weeks…climates that are for the most part warm and moist…and the flu is alive and well in this region of the world…in fact it is flourishing.  I have done a tremendous amount of reading as to why flu moves around the globe as it does.  There is the long held belief that in the temperate climates, influenza is seasonal: infections in the northern hemisphere are from November through March, and in the southern hemisphere from May to September.  One of my favorite blogs – virology.ws – had a great piece on this very issue today.

Cold and Dry or Warm and Humid?

The results of experiments in guinea pigs have revealed that aerosol transmission of influenza virus is most efficient in cold and dry conditions, and completely blocked at warm or humid conditions. Therefore it has been suggested that seasonality of influenza in temperate climates is regulated by temperature and humidity. But on a personal note here…excuse me…I am here in the warm and humid parts of the world and flu is flourishing!

In the tropics, influenza activity is more sporadic, occurring throughout the year or in temporally irregular outbreaks. One hypothesis to explain this pattern is that transmission in these areas occurs by contact, not by aerosol.

To determine if influenza transmission by contact is influenced by humidity and temperature infected and uninfected guinea pigs were housed in the same cage. The effect of environmental conditions of spread of virus to uninfected animals was then determined. The results show that temperature or relative humidity has little effect on the efficiency of transmission by contact. Based on the results obtained in guinea pigs, the authors suggest that transmission of influenza viruses in the tropics is mainly through direct or indirect contact.

The Guinea Pig Study Suggested That Contact Was The Culprit!
The Guinea Pig Study Suggested That Direct or Indirect Contact Was the Culprit!

Using the guinea pig as a model host, researchers, Anice C. Lowen, Samira Mubareka, John Steel, and Peter Palese show that aerosol spread of influenza virus is dependent upon both ambient relative humidity and temperature. Twenty experiments performed at relative humidity from 20% to 80% and 5 °C, 20 °C, or 30 °C indicated that both cold and dry conditions favor transmission.  The authors point out that a two-year study in Nicaragua revealed that influenza shows a characteristic seasonality with a peak in the middle of the rainy season.

The Nicaragua Study Revealed That Influenza Shows A Characteristic Seasonality With A Peak In The Middle Of The Rainy Season
The Nicaragua Study Revealed That Influenza Shows A Characteristic Seasonality With A Peak In The Middle Of The Rainy Season

If low temperature and humidity favor transmission of influenza, why has the 2009 swine-origin H1N1 influenza virus continued to spread throughout the spring and summer months in the northern hemisphere? Lowen and Palese offers four possible reasons:

  1. The increased number of summer cases is a consequence of increased surveillance and testing of patients with influenza-like illness.
  2. The 2009 swine-origin H1N1 influenza virus has a unique ability to transmit by the aerosol route even in warm or humid conditions.
  3. The 2009 swine-origin H1N1 influenza virus can be transmitted by aerosol under hot and humid conditions because humans have little population immunity to the virus.
  4. The virus is being transmitted mainly by contact in the northern Hemisphere and with unusually high efficiency due to the low level of population immunity.

The authors rule out explanation #1, mainly because seasonal H1N1 and H3N2 viruses that circulated in the winter of 2008-09 have not been detected frequently during the spring and summer of 2009. They believe that the remaining hypotheses are all possible, but prefer they number 4.

Contact – What is Contact?

Contact – what does that translate to?  As your Mom always said, it’s all about hygiene.  Washing hands frequently, cough hygiene and stop touching your face.

Yes, We Are Back To Hand Washing Yet Again!
Yes, We Are Back To Hand Washing Yet Again!

Our understanding of influenza seasonality has improved in recent years, but is still woefully incomplete.  We are all living in a big test tube called earth and many discoveries will likely be made as we move forward.  Our current global pandemic will be the most studied pandemic ever…hopefully some of the mysteries of influenza will be revealed under this intense scrutiny.  In the meantime, go wash your hands!


Lowen, A., & Palese, P. (2009). Transmission of influenza virus in temperate zones is predominantly by aerosol, in the tropics by contact PLoS Currents Influenza

Lowen AC, Mubareka S, Steel J, & Palese P (2007). Influenza virus transmission is dependent on relative humidity and temperature. PLoS Pathogens, 3 (10), 1470-6 PMID. http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.0030151

Do the tropics have a flu season? http://scienceblogs.com/effectmeasure/2009/03/do_the_tropics_have_a_flu_seas.php

Virology blog – http://www.virology.ws/