Assessing the severity of
an influenza pandemic
access 14 May 2009)
The major determinant of the severity of an influenza pandemic, as
measured by the number of cases of severe illness and deaths it causes, is
the inherent virulence of the virus. However, many other factors influence
the overall severity of a pandemic’s impact.
Even a pandemic virus that initially causes mild symptoms in otherwise
healthy people can be disruptive, especially under the conditions of
today’s highly mobile and closely interdependent societies. Moreover, the
same virus that causes mild illness in one country can result in much
higher morbidity and mortality in another. In addition, the inherent
virulence of the virus can change over time as the pandemic goes through
subsequent waves of national and international spread.
Properties of the virus
An influenza pandemic is caused by a virus that is either entirely new or
has not circulated recently and widely in the human population. This
creates an almost universal vulnerability to infection. While not all
people ever become infected during a pandemic, nearly all people are
susceptible to infection.
The occurrence of large numbers of people falling ill at or around the
same time is one reason why pandemics are socially and economically
disruptive, with a potential to temporarily overburden health services.
The contagiousness of the virus also influences the severity of a
pandemic’s impact, as it can increase the number of people falling ill and
needing care within a short timeframe in a given geographical area. On the
positive side, not all parts of the world, or all parts of a country, are
affected at the same time.
The contagiousness of the virus will influence the speed of spread, both
within countries and internationally. This, too, can influence severity,
as very rapid spread can undermine the capacity of governments and health
services to cope.
Pandemics usually have a concentrated adverse impact in specific age
groups. Concentrated illnesses and deaths in a young, economically
productive age group will be more disruptive to societies and economies
than when the very young or very old are most severely affected, as seen
during epidemics of seasonal influenza.
The overall vulnerability of the population can play a major role. For
example, people with underlying chronic conditions, such as cardiovascular
disease, hypertension, asthma, diabetes, rheumatoid arthritis, and several
others, are more likely to experience severe or lethal infections. The
prevalence of these conditions, combined with other factors such as
nutritional status, can influence the severity of a pandemic in a
Subsequent waves of spread
The overall severity of a pandemic is further influenced by the tendency
of pandemics to encircle the globe in at least two, sometimes three,
waves. For many reasons, the severity of subsequent waves can differ
dramatically in some or even most countries.
A distinctive feature of influenza viruses is that mutations occur
frequently and unpredictably in the eight gene segments, and especially in
the haemagglutinin gene. The emergence of an inherently more virulent
virus during the course of a pandemic can never be ruled out.
Different patterns of spread can also influence the severity of subsequent
waves. For example, if schoolchildren are mainly affected in the first
wave, the elderly can bear the brunt of illness during the second wave,
with higher mortality seen because of the greater vulnerability of elderly
During the previous century, the 1918 pandemic began mild and returned,
within six months, in a much more lethal form. The pandemic that began in
1957 started mild, and returned in a somewhat more severe form, though
significantly less devastating than seen in 1918. The 1968 pandemic began
relatively mild, with sporadic cases prior to the first wave, and remained
mild in its second wave in most, but not all, countries.
Capacity to respond
Finally, the quality of health services influences the impact of any
pandemic. The same virus that causes only mild symptoms in countries with
strong health systems can be devastating in other countries where health
systems are weak, supplies of medicines, including antibiotics, are
limited or frequently interrupted, and hospitals are crowded, poorly
equipped, and under-staffed.
Assessment of the current situation
To date, the following observations can be made, specifically about the
H1N1 virus, and more generally about the vulnerability of the world
population. Observations specific to H1N1 are preliminary, based on
limited data in only a few countries.
The H1N1 virus strain causing the current outbreaks is a new virus that
has not been seen previously in either humans or animals. Although firm
conclusions cannot be reached at present, scientists anticipate that
pre-existing immunity to the virus will be low or non-existent, or largely
confined to older population groups.
H1N1 appears to be more contagious than seasonal influenza. The
secondary attack rate of seasonal influenza ranges from 5% to 15%. Current
estimates of the secondary attack rate of H1N1 range from 22% to 33%.
With the exception of the outbreak in Mexico, which is still not fully
understood, the H1N1 virus tends to cause very mild illness in otherwise
healthy people. Outside Mexico, nearly all cases of illness, and all
deaths, have been detected in people with underlying chronic conditions.
In the two largest and best documented outbreaks to date, in Mexico and
the United States of America, a younger age group has been affected than
seen during seasonal epidemics of influenza. Though cases have been
confirmed in all age groups, from infants to the elderly, the youth of
patients with severe or lethal infections is a striking feature of these
In terms of population vulnerability, the tendency of the H1N1 virus to
cause more severe and lethal infections in people with underlying
conditions is of particular concern.
For several reasons, the prevalence of chronic diseases has risen
dramatically since 1968, when the last pandemic of the previous century
occurred. The geographical distribution of these diseases, once considered
the close companions of affluent societies, has likewise shifted
dramatically. Today, WHO estimates that 85% of the burden of chronic
diseases is now concentrated in low- and middle-income countries. In these
countries, chronic diseases show an earlier average age of onset than seen
in more affluent parts of the world.
In these early days of the outbreaks, some scientists speculate that the
full clinical spectrum of disease caused by H1N1 will not become apparent
until the virus is more widespread. This, too, could alter the current
disease picture, which is overwhelmingly mild outside Mexico.
Apart from the intrinsic mutability of influenza viruses, other factors
could alter the severity of current disease patterns, though in completely
unknowable ways, if the virus continues to spread.
Scientists are concerned about possible changes that could take place as
the virus spreads to the southern hemisphere and encounters currently
circulating human viruses as the normal influenza season in that
The fact that the H5N1 avian influenza virus is firmly established in
poultry in some parts of the world is another cause for concern. No one
can predict how the H5N1 virus will behave under the pressure of a
pandemic. At present, H5N1 is an animal virus that does not spread easily
to humans and only very rarely transmits directly from one person to