Predicting Pandemics

Thanks for posting this. I was just about ready to send you private message asking where you have been. Seems like I didn't see any post by you for a while.

Swine flu: features of past pandemics

http://scienceblogs.com/effectmeasure/2009/05/swine_flu_lessons_from_past_pa.php

... In their NEJM article Miller et al. summarize what they see as some common features in the three flu pandemics of the last century (so the generalization that there are no generalizations about flu pandemics may have some exceptions; I won't pursue the paradoxes that result). The pertinence for the current swine flu outbreak is striking.

The authors suggest five signature features characterizing pandemic versus seasonal influenza:

emergence of a flu virus with which the general population has little previous experience

peak mortality shifts to younger age groups than seasonal flu

multiple waves

enhanced transmissibility

significant geographic variation

Swine flu has already satisfied the first. It is a novel virus to which most people are not believed to have acquired immunity, although the possibility remains that those of us born before 1957 may have some cross-reactivity.

This relates to the second feature. The relative sparing of the older age groups and the increased mortality in younger adults may be related to "antigenic cycling," immune cross-reaction with a "new" virus by previous experience among the oldest in the population....It is pertinent that the age distribution of swine flu cases is markedly left-shifted (i.e., shifted to younger age groups).

The age distribution also has obvious implications for how to use an initially scarce vaccine.

The third feature, a pattern of multiple waves, characterized all three 20th-century pandemics, each of which caused increased mortality for 2 to 5 years ...The reasons for multiple waves of varying impact are not precisely understood, but they probably include adaptation of the virus to its new host, demographic or geographic variation, seasonality, and the overall immunity of the population. The occurrence of multiple waves potentially provides time for health authorities to implement control strategies for successive waves.

The Editors of Effect Measure are senior public health scientists and practitioners. Paul Revere was a member of the first local Board of Health in the United States (Boston, 1799). The Editors sign their posts "Revere" to recognize the public service of a professional forerunner better known for other things.

Oh wow this is great stuff. Some of it has occurred to me. For instance, these strains that occur so late in the season that they can't be incorporated into the next years flu vaccine might be a natural evolutionary adaptation. Evolution frequently takes the path of least resistance, if you squeeze any group of living creatures at one end of it's living space it will pop out the other end. That is what the virus seems like it is doing.

Swine Flu Is as Severe as Pandemic Virus in 1957, Study Shows

http://www.bloomberg.com/apps/news?pid=newsarchive&sid=aYOW.Fe5O3Zc

About four of 1,000 people infected with the new H1N1 strain in Mexico by late April died, according to a study published yesterday in the journal Science that was led by Neil Ferguson of the Imperial College London.

The virus is more contagious than seasonal flu, the Geneva-based WHO said yesterday. A “moderate” pandemic like the 1957 Asian flu could kill 14.2 million people and shave 2 percent from the global economy in the first year, the World Bank said in October.

“While substantial uncertainty remains, clinical severity appears less than that seen in 1918 but comparable with that seen in 1957,” the Science study authors wrote.

Swine flu has been confirmed in 4,694 people, according to the WHO, the health agency of the United Nations. Sixty-one people have died, including 56 in Mexico, three in the U.S., and one each in Canada and Costa Rica, health officials said. The U.S. confirmed 2,618 cases in 44 states, according to the Centers for Disease Control and Prevention.

Each person infected with swine flu in Mexico in April gave it to 1.4 more people on average, the study said. While that’s in the lower range of transmission speed for a pandemic virus, it’s quicker than most seasonal flus, the authors said.

In seasonal flu, each person who comes in contact with someone who’s sick has a 5 percent to 15 percent probability of illness, according to a statement on the WHO’s Web site. In swine flu, the probability increases to 22 percent to 33 percent, WHO said.

Swine flu has been “overwhelmingly mild outside Mexico,” the WHO statement said. The reason for that variation “is still not fully understood,” it said.

Swine flu is making more young people seriously ill, compared with seasonal flu, and “is of particular concern” because it’s causing more significant medical effects in people with other health conditions, the WHO said.

Assessing the severity of an influenza pandemic

http://www.who.int/csr/disease/swineflu/assess/disease_swineflu_assess_20090511/en/index.html

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.

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.

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 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 people.

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.

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 early outbreaks.

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.

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 hemisphere begins.

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 another.