Pandemic waves

An article in the_ Journal of the American Medical Association_ has questioned whether the prevailing belief that the current pandemic flu will return in a more dangerous second wave is justified. The authors also suggest that the second wave of the 1918 pandemic flu was less serious than first thought.

They say that unproven assumptions about the course of the 1918 Spanish flu pandemic may be leading to misconceptions of what the swine flu virus may do. They say that if summer weather in the northern hemisphere slows the spread of the virus, then when a second increase in cases (the wave) arrives in the autumn/winter, it is by no means certain that any increases in transmissibility or complication rates will occur.

The key points of the article are:

• Based on the pattern of spread for 14 different influenza pandemics over the past 500 years, there is little evidence to support the likelihood of a second or third more lethal wave of H1N1 influenza.
• The term “wave” entered into common use after the influenza pandemic that spread from Asia in 1889. Between 1890 and 1894 there were as many as four annual, seasonal peaks in mortality from flu reported after the main pandemic had passed.
• The 1918 influenza pandemic was thought to have killed 50 million people worldwide. However, there is little evidence that the outbreak started with a first wave of milder illness followed by a second, more deadly wave when the virus mutated into a more transmissible and virulent form.
• There is also little convincing evidence from the 1957 and 1968 pandemics to support the idea that viruses start out relatively mild before turning into more lethal mutations.

Where was the article published?

Dr David M Morens and Dr Jeffery K Taubenberger from the US National Institute of Allergy and Infectious Diseases published this commentary in the Journal of the American Medical Association.

What does it say?

The authors say that the circulation of the swine flu virus in the northern hemisphere during the spring of 2009 has led to inevitable comparisons with the 1918 flu pandemic. In this commentary, they question this and say that changes in virulence or transmissibility of the current pandemic virus are not inevitable.

The authors state that there is a long-standing theory that as new viruses start to circulate in human populations, they mutate into versions with increased transmissibility and virulence. The prevailing scientific view is that the 1918 spring outbreaks of flu were mainly mild disease and represented a so-called “herald wave”. Scientists believe the virus then mutated over the summer, making the disease it caused more severe when it returned. This pattern of herald waves followed by seasonal waves was demonstrated in influenza and dengue fever in the early part of last century, around 1918. The authors say this theory is behind much of the aggressive public response to pandemics, and explains some of the planning assumptions behind many countries’ preparedness plans.

However, there are no virus samples from the spring wave of 1918 flu pandemic, the authors state, so there is no way to confirm that later outbreaks were caused by the same virus or that the virus changed to become more virulent. They say that it is speculation to assume what happened and that research to determine the true course of events has not been carried out or is confusing.

For example, they say the first documented wave of 1918 is often confusingly referred to as the “spring wave”, when it was actually a summer surge of influenza deaths concentrated in some, but not all, northern European countries between late June and August 1918. Additionally, they say that it is curious that, despite enormous wartime traffic, many English cities had a summer wave but France did not.

What they find most puzzling is that, during the 1918 pandemic, different countries had anything up to three waves and the course or timing of these in different countries in the same hemisphere varied greatly.

What do the researchers say?

Dr Taubenberger said, "I think every pandemic is completely different, it emerges in a different way. Its genetics are going to be different and the population immunity by age is going to be different depending on what the virus is. So I think it's very difficult to assume that a new pandemic is going to behave in a way like 1918."

The authors also say that there is a general tendency for pandemics quickly to assume annual seasonality in temperate zones. This means that the difference between a seasonal post-pandemic recurrence and a seasonal endemic recurrence seems to blur over time. This occurs as the immunity to the virus in the population increases and as the antigen composition (the surface molecules) of the viruses gradually change (drift) over time.

What is the implication and importance of this?

The authors acknowledge the difficulty of predicting the future course of this pandemic. They admit there is always a chance, however small, that the swine flu virus could mutate, with serious consequences for public health. This is the reason behind the prudent approach taken by international public health bodies.

They conclude by advising readers to remember that, as Kierkegaard said about life, influenza epidemics are lived forwards and understood backwards.