Influenza Virus

Influenza, commonly called “the flu,” is a viral illness that infects the respiratory tract of many animals, birds, and humans. It is divided into three types, namely A, B, and C. Furthermore, the differences between two specific proteins in the capsular wall of the influenza A virus provide the basis of their classification into subtypes. eg. A/Hong Kong/1/68 (H3N2) signifies an influenza A virus isolated from a patient originating from Hong Kong  in 1968, and of subtype H3N2.
Influenza types A and B are responsible for epidemics of respiratory illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C differs from types A and B and usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public-health impact of influenza types A and B. Efforts to control the impact of influenza are aimed at types A and B.
Influenza viruses are continuously changing, usually by mutation of their genetic makeup. Influenza type A viruses undergo two kinds of changes. One is a series of mutations that occurs over time and causes a gradual evolution of the virus. This is called antigenic “drift.” The other kind of change is an abrupt change. This is called antigenic “shift.” In this case, a new subtype of the virus suddenly emerges. Influenza type B viruses change only by the more gradual process of antigenic drift and therefore do not cause pandemics. The most widely held view for the cause of this “shift” is that the new virus subtypes are reassortant viruses resulting from double infection, so that that 8 RNA segments of each strain reassort with each other, producing a new virus.
This constant changing often enables the virus to evade the immune system of the host (humans, birds, and other animals) so that the host becomes susceptible to the changed influenza virus resulting in infection, hence the need for yearly vaccination.

Most of us think that epidemic and pandemic are synonymous. But, they are not. Simply put, when an epidemic gets out of hand, it is called a pandemic. This has 2 implications:

  • Geographical spread : An epidemic that is not localized to a city or a small region but spans a larger geographical area can be called a pandemic.

    Incidence rate : An epidemic may be localized to a small region but the number of people affected may be very, very large compared to what is “expected”. In this case, it can be called a pandemic even if its geographical spread is not very large. For example, let us say that a disease has an “expected” rate of infection of 15%. When 40% of the population of a state is infected, we have an epidemic on our hands. When 75% of the population is infected, it has reached pandemic proportions.

This “expected” rate of infection for a given disease which occurs with regular consistency at specific times of the year is called a seasonal infection. Epidemics may appear at any time but are usually concentrated in months of high humidity. They occur explosively with little or no warning. The number of people affected can vary from a few hundred to hundreds of thousands. Epidemics may be short-lived, lasting days or weeks but larger epidemics may last for months.

Unusually severe worldwide outbreaks (pandemics) have occurred several times in the last hundred years since influenza virus was identified. The term “influenza” was first coined by an Italian in the mid-1700’s to denote a disease resulting from miasma (bad air). The human disease is thought to have arisen about 6000 years ago. A human influenza virus was not isolated until 1933.
Since the first documented Spanish Flu Pandemic of 1918-19, pandemics continued to occur regularly, namely in 1932-33, 1947-48, 1957, 1968, and 2009. These latter pandemics resembled the pandemic of 1890, affecting millions of people with a mild URTI (upper respiratory tract infection) and a small number of deaths.

The H1N1 viruses (so-called “swine” viruses as their main host s are pigs)  probably first appeared (in pandemic form) in 1918 and continued to circulate until 1957, at which time they were supplanted by the H2N2 (Asian) viruses. The H2N2 viruses were prevalent until 1968, when H3N2 (Hong Kong) strains appeared. The H1N1 virus reappeared in 1977 and did not replace the H3N2 subtype and both subtypes continued to cocirculate. In 2009, a novel strain of H1N1 appeared that was distinct from the prevailing H1N1 strain that appeared in 1997. It was thought to be a triple reassortant mutation between human, bird, N. American and Eurasian pig influenza viruses.
This new influenza strain against which the world population had little or no immunity was isolated from humans in April 2009 in Mexico. It spread throughout the world so fast that the WHO declared this new flu strain (termed novel H1N1 influenza A swine flu, often shortened to H1N1 or swine flu) as the cause of a pandemic on June 11, 2009. This was the first declared flu pandemic in 41 years.

After exposure to a virus, the human body (or the body of any other animal for that matter), can develop a certain immunity to it. This process works as follows: a host infected with influenza virus develops antibodies against that virus, making it immune to that virus strain. As the virus changes, the “first” antibody no longer recognizes the “newer” virus and infection can occur because the host does not recognize the new flu virus as a problem until the infection is well under way. That first antibody however, may in some instances, provide partial protection against infection with a new influenza virus. This is the usual situation for the yearly occurring “conventional” or “seasonal” flu strains. However, there are situations in which some flu outbreaks are severe. These severe outbreaks occur when the human population is exposed to a flu strain against which it has little or no immunity because the virus has become altered in a significant way. i.e. “novel” strains such as the 2009 H1N1 virus
The viruses used in making seasonal flu vaccines are chosen each year based on information collected over the previous year by 130 national influenza centers in 101 countries. This data as well as information on disease trends are further analyzed by the World Health Organization (WHO), after which the WHO recommends specific vaccine viruses for vaccine production, but then each individual country makes their own decision for licensing of vaccines in their country.
The seasonal flu vaccine is usually a trivalent vaccine (a three component vaccine) with each component selected to protect against one of the three groups of influenza viruses circulating most commonly in humans. The 2009 H1N1 vaccine that was made to protect against the pandemic virus first detected in April was a monovalent (one-component) vaccine that only protected against the 2009 H1N1 viruses. The three vaccine viruses are chosen to maximize the likelihood that the main circulating viruses during the upcoming flu season (from end of September to May) will be well covered by the vaccine. WHO has recommended that the Northern Hemisphere’s 2010–2011 seasonal influenza vaccine contain the following three vaccine viruses:

an A/California/7/2009 (H1N1)–like virus,
an A/Perth/16/2009 (H3N2)–like virus, and a
B/Brisbane/60/2008–like virus.
The H1N1 virus recommended for inclusion in the 2010-2011 seasonal influenza vaccine is a pandemic 2009 H1N1 virus and is the same vaccine virus as was used in the 2009 H1N1 monovalent vaccine.

When spread by droplets or direct contact, the virus, if not killed by the host’s immune system, replicates in the respiratory tract and damages host cells. In people who are immuno-compromised the virus can cause viral pneumonia or stress the individual’s system to such an extent that they become more susceptible to bacterial infections, especially bacterial pneumonia.
Groups at increased risk of conventional/seasonal influenza complications include:

  • People aged 65 years or older

    Residents of nursing homes and other chronic-care facilities housing patients of any age who have chronic medical conditions;

    Adults and children with chronic disorders of the pulmonary, cardiovascular, or immune systems, including children with asthma;

    Adults and children who have required regular medical follow-up or hospitalization during the preceding year because of chronic metabolic diseases (including diabetes mellitus), renal dysfunction, hemoglobinopathies, or immunosuppression (including immunosuppression caused by medications);

    Children and teenagers (6 months to 18 years of age) who are receiving long-term aspirin therapy and therefore may be at risk for developing Reye syndrome after influenza;

    Women in the third trimester of pregnancy or in the early postpartum period. There is some evidence to suggest that women who are in the third trimester of pregnancy or in the early postpartum period may be at increased risk for serious medical complications after influenza infection.

Furthermore it is advisable that all children 6-59 months of age get a yearly conventional flu vaccination since each year there are many children who require hospitalization because of the flu and flu is easily passed from child to child.
In addition, the following groups should be vaccinated because they may transmit influenza to people who are at high risk for complications if they become infected with either the conventional or novel H1N1 flu.

  • Physicians, nurses, and other health-care personnel in both hospital and outpatient-care settings

    Employees in nursing homes and chronic-care facilities who have contact with patients or residents

    Providers of home care to people at high risk (for example, visiting nurses and volunteer workers)

    Household members (including children) of high-risk people

Finally, the flu vaccine may be administered to any person who wishes to reduce his or her chances of acquiring influenza infection. People who provide essential community services should be considered for vaccination to minimize disruption during influenza outbreaks. Students or other people in institutional settings, such as those who reside in dormitories, should be encouraged to receive the vaccine to minimize the disruption of routine activities during epidemics. The only people who should not have the vaccination are those who are allergic to eggs as these are used as the culture medium to produce the vaccine or if there is a known allergic reaction to the vaccine in previous years.
Unfortunately, because the novel 2009 H1N1 flu virus did not follow the conventional flu pattern for time of disease appearance (it appeared in April and increased over the summer and autumn of 2009), or the usual susceptible populations, recommendations for vaccination have been altered this year.
Groups at increased risk of novel H1N1 influenza complications include:

  • Pregnant women,

    People who live with or care for children younger than 6 months of age,

    Health-care and emergency-services personnel,

    People between the ages of 6 months through 24 years of age and children 5-18 years of age who have chronic medical problems, and

    People from 25-64 years of age who are at higher risk for novel H1N1 because of chronic health disorders or compromised immune systems.

 Although annual influenza (injectable) vaccination has long been recommended for people in the high-risk groups, many still do not receive the vaccine, often because of their concern about side effects. They mistakenly perceive influenza as merely a nuisance and believe that the vaccine causes unpleasant side effects or that it may even cause the flu. However, injectable influenza vaccine has never been capable of causing influenza because it consists of a killed virus. Some people do not receive influenza vaccine because they believe it is not very effective. There are several different reasons for this belief. People who have received influenza vaccine may subsequently have an illness that is mistaken for influenza, and they believe that the vaccine failed to protect them. These symptoms which include mild fever and cold-like symptoms are often simply an immune-response reaction.
Overall vaccine effectiveness varies from year to year, depending upon the degree of similarity between the influenza virus strains included in the vaccine and the strain or strains that circulate during the influenza season. Because the vaccine strains must be chosen nine to 10 months before the influenza season, and because influenza viruses mutate over time, sometimes mutations occur in the circulating virus strains between the time the vaccine strains are chosen and the next influenza season ends. These mutations sometimes reduce the ability of the vaccine-induced antibody to inhibit the newly mutated virus, thereby reducing vaccine efficacy. This commonly occurs with the conventional flu vaccines as the specific virus types chosen for vaccine inclusion are based on reasoned projections for the upcoming flu season. Occasionally, the vaccine does not match the actual predominating virus strain and is not very effective in generating a specific immune response to the predominant infecting flu strain, resulting in people still becoming infected with the influenza virus despite vaccination against it.

On the 10th August, 2010, the director general of WHO, Dr Margaret Chan said H1N1 has now moved to a post-pandemic period and that the novel H1N1 virus has largely run its course. As we enter the post-pandemic period, this does not mean that the H1N1 virus has gone away. Based on experience with past pandemics, it is expected that the H1N1 virus will take on the behaviour of a seasonal influenza virus and continue to circulate for some years to come. Globally, the levels and patterns of H1N1 transmission now being seen differ significantly from what was observed during the pandemic. Out-of-season outbreaks are no longer being reported in either the northern or southern hemisphere. Influenza outbreaks, including those primarily caused by the H1N1 virus, show an intensity similar to that seen during seasonal epidemics. During the pandemic, the H1N1 virus crowded out other influenza viruses to become the dominant virus. This is no longer the case. Many countries are reporting a mix of influenza viruses, again as is typically seen during seasonal epidemics.
If you would like further information regarding the use and availability of the influenza vaccine please come and talk to us. Luzdoc will be stocking the influenza vaccination, which will have the three vaccine viruses that are recommended for 2010/2011, from September.

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