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The virus responsible for this outbreak of swine flu belongs to the type A, which comprises four main strains: H1N1, H1N2, H3N2 e H3N1. Most of the cases detected so far have been affected by the H1N1 type. H1N1 is the same strain that causes seasonal flu outbreaks in humans but the newly detected version contains genetic material from pigs and birds influenza. Some experts believe that we are not in the presence of a completely new virus, but rather a mutation of an H1N1 virus first isolated in 1933. Since then, the virus may have changed about 60-70% of its genetic code, but it’s unlikely to have mutated completely, which makes it potentially less lethal than otherwise.
According to the Mexican Health Ministry, so far 1,614 have been infected and 103 have died. Of theses, only 20 were so far confirmed to have been caused by the new virus. Most of them were is the 25-45 age range, which generally tends to identify potential pandemic threats as opposed to normal flu episodes. Those who are infected outside Mexico have experienced only mild symptoms, and no one has died yet. If the virus mutates further and become fully transmissible human-to-human, the threat can be greater, especially if characterized by a relatively low case/fatality rate.
In fact, a virus that causes less debilitating disease and fewer fatalities is likely to infect more people. For instance, the Ebola virus that killed hundreds of people few years ago in Africa (despite being highly infectious) proved to be characterized by a not very ’successful” strategy of diffusion since it used to kill its victims too fast, preventing itself from spreading more (which is the ultimate goal of a virus). A virus able to kill 5% percent, and not 50% of those infected (like the Avian flu) would be therefore more dangerous and would dramatically resemble the 1918 Spanish flu pandemic, which killed about 50 million people. Since at that time the world population was 1.8 billion people, today a pandemic with the same case/fatality rate would cause about 150 million deaths.
Pandemics appear regularly, and 40yrs have passed since the last one. The last century witnessed the outbreak of three main pandemics: (1) the Spanish flu in 1918-1919, which infected up to 40% of the world’s population and killed more than 50m people, with young adults being particularly badly affected; (2) the Asian flu in 1957 was caused by a human form of the virus H2N2, combining with a mutated strain found in ducks, and killed two million people, with the elderly being particularly vulnerable; (3) the 1968 outbreak first detected in Hong Kong, caused by a strain known as H3N2, killed up to one million people globally, with the over-65s being the most affected. During this century, the pandemic risk has been associated with two episodes: the SARS in 2003 and the avian flu, which has hit the scenes in three different waves since 2003. Let’s examine some of them for comparison.
THE SPANISH FLU: The so called Spanish flu, the most intense pandemic that have occurred to date, hit the world between 1918 and 1919 by inflecting one billion people, about 40% the world’s population at that time, and killed between 40 and 50 million (for comparison, the just finished WWI had an estimated death toll of 10 million). The given name “Spanish” comes from the fact that Spain, not being involved in the war, had a free press that could report the developments of the pandemic in their country, and for this reason when the Russian newspaper Pravda reported on the situation in Moscow, the headline was “Ispanka (The Spanish Lady) is in town”. The first town the ‘Lady’ visited was Camp Funston, Kansas – USA on 8th March 1918, and then the pandemic hit the world in 3 waves: summer 1918, autumn 1918, early 1919. Worldwide, since the second wave took only 2 months to circle the globe, it is estimated that 30% of the world’s population fell ill with influenza.
The most remarkable characteristics of this pandemic was its age distribution mortality: while general influenza claims the older and younger segments of the population, 45% of the victims of the Spanish Lady were people aged 15-35 (more than the usual 9%), and fully 50% of the mortality occurred between ages 15 and 44. What made this figure impressive is not the 2.5% of the so-called case fatality rate (deaths/infected people), but the morbidity of the disease (infected people/population) which varied between 25% and 50% across the regions, and the fact that a healthy young individual could die within 48 hours after contracting the disease. Apart from the age distribution, the Spanish flu was remarkably ‘democratic’ in its victims. First of all, there was not a noticeable sex differentiation of the victims (52% women – 48% men). Secondly, little association was found between influenza mortality and general health standards, wealth and overcrowding conditions. Crowding was not an issue because the virus was so infectious that the likelihood of the exposure to the virus of all persons living under urban conditions was just negligibly increased by an increase in the number of people living in the same dwelling. Thirdly, all in all, the incidence upon town and country was nearly equal, with the towns not suffering more than the countryside, but suffering first. The diffusion was in fact helped by the movements of troops at that time traveling through many ports simultaneously. However some studies suggest that urban areas, coastal areas and areas well served by mass communication and transport link suffered higher mortality rate than rural inland and isolated areas.
SARS: This virus which was declared a pandemic in March 2003 and the most affected countries were Singapore, Hong Kong, China and Malaysia. Other countries involved were Canada, South Africa, Sweden, France and the United States. SARS caused some 8,000 cases and close to 800 died from the virus.
THE AVIAN FLU: As we mentioned, the Avian Flu (or Bird flu) hit the world in three waves. The first wave of H5N1 (the so-called Yunnan) outbreaks occurred in late 2003 and 2004 in many Asian countries. The second wave (called Guiyang) started in China’s Qinghai Lake in May 2005 and the strain characterizing that wave is the one that has been found in parts of Europe, Africa and the Middle East. A third wave started in October 2005, led by a strain called the “Fujian-like virus” because it was first isolated in China’s southern Fujian province in March 2005. The strain proved to be resistant to the vaccines that China began using on a large scale from September 2005 to protect poultry from H5N1.
The new strain had infected poultry in Hong Kong, Laos, Malaysia and Thailand, and sickened people in China and Thailand. Since its first appearance, the three strains of H5N1 virus are reported to have killed more than 209 million poultry worldwide, the UN’s Food and Agriculture Organization said in a report released in June 2007. However, despite being strongly infectious for birds, (luckily) the virus proved to be still poorly infectious in humans. According to a WHO report released on Oct ‘07, the H5N1 virus is known to have infected 256 people in 10 countries in the past three years, killing 152 of them. More than five of every 10 reported cases were fatal, implying a case fatality rate (deaths/infected people) of 50%, much higher that the 2.5% of the Spanish flu.
রাজিব দা, আপনি যদি বাংলায় আপনার এই ব্লগটা চালান, বাংলাদেশের অনেক মানুষ উপকৃত হত। একটু ভেবে দেখবেন?