swine fleu n1h1 virus characteristics

Virus characteristics


The virus is a novel strain of influenza. Existing vaccines against seasonal flu provide no protection, and there is no vaccine for this strain. A study at the U.S. Centers for Disease Control and Prevention published in May 2009 found that children had no preexisting immunity to the new strain but that adults, particularly those over 60, had some degree of immunity. Children showed no cross-reactive antibody reaction to the new strain, adults aged 18 to 64 had 6-9%, and older adults 33%.[169][170] It was also determined that the strain contained genes from four different flu viruses: North American swine influenza, North American avian influenza, human influenza, and two swine influenza viruses typically found in Asia and Europe. Further analysis showed that several of the proteins of the virus are most similar to strains that cause mild symptoms in humans, leading virologist Wendy Barclay to suggest on May 1 that the initial indications are that the virus was unlikely to cause severe symptoms for most people.[171] Scientists in Winnipeg completed the first full genetic sequencing of the virus on 6 May.[172]

Virus origins

On June 23, the New York Times reported that U.S. federal agriculture officials, "contrary to the popular assumption that the new swine flu pandemic arose on factory farms in Mexico," now believe that it "most likely emerged in pigs in Asia, but then traveled to North America in a human." They emphasized that there was no way to prove their theory, but stated that there is no evidence that this new virus, which combines Eurasian and North American genes, has ever circulated in North American pigs, "while there is tantalizing evidence that a closely related 'sister virus' has circulated in Asia."[141]
In early June 2009, using computational methods developed over the last ten years at Oxford, Dr Oliver Pybus of Oxford University's Department of Zoology and his team attempted to reconstruct the origins and timescale of the 2009 flu pandemic. He claims "Our results show that this strain has been circulating among pigs, possibly among multiple continents, for many years prior to its transmission to humans." The research team that worked on this report also believe that it was "derived from several viruses circulating in swine," and that the initial transmission to humans occurred several months before recognition of the outbreak. The team concluded that "despite widespread influenza surveillance in humans, the lack of systematic swine surveillance allowed for the undetected persistence and evolution of this potentially pandemic strain for many years."[173]


Structure of the influenza virion. The hemagglutinin (HA) and neuraminidase (NA) proteins are shown on the surface of the particle. The viral RNAs that make up the genome are shown as red coils inside the particle and bound to ribonucleoproteins (RNPs).
According to the researchers, movement of live pigs between Eurasia and North America "seems to have facilitated the mixing of diverse swine influenza viruses, leading to the multiple reassortment events associated with the genesis of the (new H1N1) strain." They also stated that this new pandemic "provides further evidence of the role of domestic pigs in the ecosystem of influenza A."[135] Some experts also suspect that a leading cause of the new virus outbreak is insufficient surveillance by the pork industry and the fact that "animal husbandry now more closely resembles the petrochemical industry than the happy family farm"[174]
Estimates of total cases
On July 24, Keiji Fukuda, WHO's Assistant Director-General for Health Security and Environment said the pandemic was still in its early stages globally and two billion infections over the course of the pandemic was "a reasonable ballpark to be looking at."[175] On the same day, the Anne Schuchat, director of the U.S. CDC's National Center for Immunization and Respiratory Diseases, estimated that without a swine flu vaccine, current trends suggest 12% to 24% of Americans might get swine flu this fall and winter. But if large enough numbers of people get the vaccine before the next waves of the pandemic sweep the nation it would "put a dent in the pandemic," she said. Initial availability of vaccines is expected by mid-October, although healhcare workers and people most at risk would likely get them first.[176]
As for the current totals, in early July WHO officials gave up trying to count the number of cases, saying the best they can do is estimate the spread of the "unstoppable" virus. And the CDC's Schuchat declined even to hazard a guess, saying it was "more than a million." Officials do not have the tools they need to count, she pointed out. "Most people who have respiratory illnesses don't find out exactly what caused it. Even most people with influenza don't know exactly which type of influenza caused their illness."[177]
In late June, the CDC estimated that 1 million Americans had so far contracted the flu. By comparison, an estimated 15 million to 60 million Americans are infected with the seasonal flu each year, leading to roughly 36,000 deaths.[178] According to the CDC, however, only about one in 20 cases was being officially reported in the U.S. in late May.[179] In the U.K., some experts thought the number of cases was potentially 300 times more than early published estimates,[180] warning that case estimates by the U.K. and other governments were "meaningless" and hiding its true extent. There were also estimates that Japan may have had approximately 30,000 cases by late May.[180]

Virulence

The CDC has noted that most infections continue to be mild—similar to seasonal flu—and recovery is extremely quick.[181] Some experts point out that the deaths so far are "a tiny fraction" of people who die every year from seasonal flu - with barely a public murmur." One doctor said that "when there's something that's new and unknown, it scares people."[182] Some medical journalists suggest that the news media may be overreacting to the new virus and have not compared its virulence with that of the regular "seasonal" flu.[183]
Mutation potential
On May 22, 2009, WHO chief Dr. Margaret Chan said that the virus must be closely monitored in the southern hemisphere, as it could mix with ordinary seasonal influenza and change in unpredictable ways. "In cases where the H1N1 virus is widespread and circulating within the general community, countries must expect to see more cases of severe and fatal infections," she said. "[184] Experts writing in the July New England Journal of Medicine note that "historically, pandemic viruses have evolved between seasons, and the current strain may become more severe or transmissible in the coming months." They therefore stress that "international cooperation will be crucial" to engage in proper surveillance to help monitor changes in the virus's behavior, which will aid in both "vaccine targeting" and interpreting illness patterns in the fall of 2009.[143]
Other experts are also concerned that the new virus strain could mutate over the coming months. Guan Yi, a leading virologist from the University of Hong Kong, for instance, described the new H1N1 influenza virus as "very unstable", meaning it could mix and swap genetic material when exposed to other viruses. During an interview he said "Both H1N1 and H5N1 are unstable so the chances of them exchanging genetic material are higher, whereas a stable (seasonal flu) virus is less likely to take on genetic material." The H5N1 virus is mostly limited to birds, but in rare cases when it infects humans it has a mortality rate of between 60% to 70%.[185] Experts worry about the emergence of a hybrid of the more virulent Asian-lineage HPAI (highly pathogenic avian influenza) A/H5N1 strain (media labeled "bird flu") with more human-transmissible Influenza A strains such as this novel 2009 swine-origin A/H1N1 strain (media labeled "swine flu"), especially since the H5N1 strain is and has been for years endemic in birds in countries like China, Indonesia, Vietnam and Egypt.[185][186] (See the suite of H5N1 articles for details.)
Nor had federal health officials in the U.S. dismissed the possibility that the worst was yet to come. "Far from it," Ann Schuchat of the CDC says, noting that the horrific 1918 flu epidemic, which killed hundreds of thousands in the United States alone, was preceded by a mild "herald" wave of cases in the spring, followed by devastating waves of illness in the autumn. "That 1918 experience is in our minds," she said.[187]
As of late July, U.S. health officials said that the swine flu "isn't yet mutating to become more dangerous," but they are closely tracking that as the virus continues to circle the globe.