Introduction
Influenza is an acute and very contagious disease that spreads rapidly, causing serious problems in terms of morbidity and mortality, which are brought about, among other factors, by the antigenic variability of the viruses that cause it. Described by Hippocrates around the year 400 B.C., we find accounts about “epidemic catarrhs associated with seasonal periods”, which were probably episodes of what today goes by the name of influenza. A number of authors reckon that “Athen’s plague” described by Thucydides may in fact have been a flu epidemic. In Florence, in the 14th century, this disease started to be named “influenza” (“of cold”, influence of cold, “of the stars”, influence of the stars). English scientists named it “influenza” and subsequently, in France, it was called “grippe”, term from which the Spanish name derives. One of the first clinical descriptions is likely to date back to 1510, when nosebleeds, pneumonias and serious conditions for pregnant women where reported. This disease may develop in various forms: sporadic, epidemic or pandemic. Between the 12th and the 18th century, literature has reported a number of pandemics, among which the 1889-91 pandemic stands out: this started in Siberia and subsequently spread to Russia and to the rest of Europe. But the most recent pandemics occurred in the 20th century and caused numerous deaths among people of all ages. The main pandemic of the 20th century is known as “Spanish Flu” (1918-19), and caused over 20 million deaths; it started during the Great War and it developed in three stages: spring 1918 (“normal” pandemic), autumn of the same year (“killer” pandemic) and spring 1919, with a decline in negative effects. The outbreak of the pandemic took place on the European Western Front and for a certain period it was kept secret to keep up the spirits within the armies. During the same period, “highly virulent” flues had appeared in Western Africa (Sierra Leone) and China, where the pandemic probably started. Other 20th century pandemics occurred in 1957 and 1968. The 1957 pandemic (“Asiatic Flu”) with H2N2 subtypes, spread in two waves in 1957 and 1958, with high morbidity and low mortality rates. In 1968 another pandemic, with H3N2 subtypes, known as “Hong-Kong Flu”, spread in two waves, the latter being the most virulent. Lastly, in 1977, the A H1N1 virus (“Russian Flu”) reappeared with a lower morbidity-mortality rate.

Underlying Biological Factors
The etiology of influenza remained unknown for a long time, but researchers made considerable progress during the 1889-1890 epidemic, when Pfeifer in Germany isolated a bacterium that prevailed in the victims’ lungs (a gram-negative bacillus corresponding to haemophilus influenzae). Subsequently, a similar bacterium was found in swine influenza: haemophilus influenzae suis. Hannoun reported that, in those days, a senior intern of the Pasteur Institute, Dujarric, expounded a theory according to which the agent responsible for influenza was a “filterable virus”. It was thanks to this person and to another researcher on the subject, Nicolle, that great progress was made on influenza etiology during the 1918 epidemic. A few years later, Shope, in the United States, discovered that a virus and a bacterium were involved in swine influenza. In 1933, Smith, Andrewes and Laidlaw were to isolate the first virus of human influenza.

Influenza Viruses
Influenza is caused by various viruses: A, B and C; these all belong to the orthomyxovirus family; it should be kept in mind that virus A and B cause human epidemics and that immunity against one does not involve immunity against the others. The influenza type A virus is widely present in nature and it can infect a great variety of animal species, including man. It has caused a number of pandemics. The influenza type B virus, which can only be carried by man, produces infections displaying the same features as virus A. Virus C plays a minor role in causing the disease. All influenza viruses are medium-sized RNA viruses and display an external membrane surrounding the nucleocapsid.

The membrane is made up of lipids and its internal surface is covered with a proteinic matrix (protein M1). The external surface is covered with glycoprotein spicoles: these are the chief antigenic structures of the virus and they correspond to hemagglutinin and to neuroaminidase. The above mentioned surface antigens, hemagglutinin and neuroaminidase, display 15 subtypes (H1-H15) and 9 subtypes (N1-N9) respectively. Hemagglutinin is the most important antigen, since it has specific features depending on subtype and strain, or variant, it allows the virus to adhere to the membrane and it is important for the infection to develop. Neuroaminidase is a specific subtype antigen, it contributes to the release of viral particles from infected cells and fosters adhesion of the virus to the host’s cells. The chief subtypes detected in the researches carried out during the various pandemics are H1, H2, H3 and N1 and N2. (H1N1 (1947); H2N2 (1957); H3N2 (1968) or reappearance of H1N1 (1977). Certain animals suffer from influenza affections (mammals, domestic and migratory birds) and various evidences suggest that certain pandemics are associated with viruses of an animal origin (1918, swine strain; 1957 and 1968, avian strains). More recently we have witnessed the appearance of various Virus A subtypes, such as (H5N1) in 1997 or (H9N2) in 1999, both in Hong Kong, which luckily have not resulted in pandemics.

The nucleocapsid contains: RNA segments (8 filaments codifying different proteins), nucleoproteins (which classify viruses into A, B and C) and polymerase.

Antigenic Variations: Influenza viruses may display various degrees of variations: these can be classified as minor variations, named ‘drifts’, or major variations, named ‘shifts’. Minor variations relate to hemagglutinin and lead to the appearance of new strains; these take place as a result of spontaneous gene mutations, which codify hemagglutinin and bring about new epidemic events. Major variations are known as total mutations of the H or N antigen, or of both, which lead to the appearance of pandemics.

Pathogenesis
Influenza is a highly infective and contagious disease. The virus attacks individuals through the respiratory system, through cough and sneezes containing infective particles. This event is followed by a viral multiplication phase, with a short viraemia period during the course of the first infection. The virus triggers sensitisation phenomena with an immune response (activation of cytotoxic T cells and of B lymphocytes). The disease, which is very contagious, may display a classic symptomatology, with general ailment, high fever, rhinorrhea, myalgia and lack of appetite. In many cases the problem will be solved, whereas in others it may develop into primary viral pneumonia. In certain cases, complications may occur with the development of bacterial superinfections. In interpandemic periods there are few fulminant cases among young people, which is not the case during great pandemics.

Epidemiology
Influenza is the main cause of acute respiratory diseases at all ages. The influenza virus circulates all the year round; however, it is a seasonal disease and it prevalently attacks between December and April in the northern hemisphere and between May and September in the Southern hemisphere. In the Tropics, epidemics are likely to appear during the wet season. Epidemics, which are caused by minor variations, involve an increase in the morbidity-mortality rate, although it is in fact during great pandemics, due to greater virus mutations, that morbidity increases (over 45-50% of the European population was infected with the 1918 flu) and at times mortality rates also increase (3% during the same period, with over 20 million deaths). As we have already mentioned, the chief negative effects are mainly represented by pneumonia (primary or secondary) accompanied by an increase in the hospitalisation rate, with a growth in elderly people’s mortality rate. In countries offering generally good hospitalisation facilities for influenza, the hospitalisation rate ranges between 200 and 1000 / 100,000 inhabitants, among people over 65 (with or without risk factors) and between 100 and 500 / 100,000 inhabitants for the 0 - 4 age bracket. If we analyse mortality caused by influenza, in most studies 96%-98% of casualties is reported to relate to people over 60.

Epidemics and Complications
All epidemiological data relating to the flu take place in conjunction with the above-mentioned circumstances, which lead to the appearance of epidemics (minor virus mutations) and are:
- Increase in the number of primary and secondary attacks due to the disease.
- Increase in medical examinations
- Increased activity and collapse of first-aid stations
- Increased hospitalisation rate
- Increase in complications (primary and secondary pneumonia, EPOC aggravation, bronchiolitis, otitis, myocarditis, etc.)
- Increased mortality for the various groups at risk

Groups at Risk
All individuals exposed to the influenza virus may develop the disease, but there are a number of risk factors that increase the probabilities of contracting it and these can be grouped as follows:
- People over 65
- Guests of old people’s homes
- Children and adults, irrespective of patient’s age, suffering from chronic diseases (cardiovascular, pulmonary – including asthma - renal diseases, or diabetes).
- Immunodepression problems (natural or drug-related)
- Prolonged aspirin treatments in children and teenagers in the 6 months – 18 years age bracket, owing to the possibility of developing post-influenza Reye’s Syndrome
- Pregnant women, if the second or third pregnancy trimester coincides with the influenza epidemic period.
- Women during beast feeding
- Groups of people belonging to collectives that may easily spread the virus:
* Hospital workers, first aid or domiciliary care staff
* Voluntary workers
* Social workers Other exposed groups are:
* HIV positive individuals (vaccination is indicated in patients affected by AIDS with normal or low CD4 count, even though a transitory replication of HIV may occasionally appear during the 2-4 weeks following vaccination).
* Travellers

Prevention
The best way of preventing this disease and reducing its diffusion is, in addition to healthcare education, vaccine administration. The indication to vaccinate against influenza is aimed at:
- Preventing virus circulation (vaccination of school-age children and of old people’s home guests)
- Preventing complications and overmortality (remaining part of recommendations coinciding with those of other countries).
The changeable nature of the virus makes it necessary to vaccinate individuals at risk on a yearly basis and therefore, each year, the Worldwide Health Organisation (WHO) organises two meetings for influenza experts to indicate which strains ought to be used for the vaccines. The meetings are organised in February for the northern hemisphere and in September for the southern hemisphere.

Vaccine Types
One of the main features of the influenza viruses is their ability to elude the host’s immune response, by causing annual epidemic events, some of which bring about actual pandemics. The WHO conducts its surveillance activity on each virus’ responsibility all the year round, in the attempt to detect the mutations of the influenza virus A and B, and guarantee, as mentioned above, the recommended composition of the annual vaccine. In this regard, the market has available various vaccine types. They all contain three strains, two strains of the type A virus and one strain of the type B virus (strains recommended by the WHO) which are announced each year in February for the northern hemisphere and in September for the southern hemisphere. Flu vaccines have evolved over the years. First-generation vaccines (1950s), formed by whole viruses, were composed of viruses inactivated through chemical processes. During the following decade (60s), second-generation vaccines appeared, with split-viruses disintegrated by means of solvents or detergents. In 1976 third-generation vaccines, or subunit vaccines, were marketed: these were composed of purified surface antigens. Lastly, as from 1997, we have fourth-generation vaccines, which are subunit vaccines fixed to an adjuvant that strengthens the immune response. Most vaccines are prepared with inactivated viruses cultured in eggs, and in many of these mercury thiosalicylate is used as a preservative.

All vaccines have similar positive effects, although they display some limitations, such as: -Slowness in the appearance of immunity (2 - 3 weeks)
-Short immunisation period (8-12 months)
-Need for the vaccination to be repeated annually
-A more limited response in people with an altered immune system
-Limited immune response in elderly people

Composition, Doses and Models
Controls carried out by the WHO through associated laboratories have detected between October 2000 and February 2001 the circulation of a type A virus (H1N1) antigenically similar to the previous campaign vaccine strain and, sporadically, a type A virus (H3N2) similar to the A/ Moscow/10/99 and to the previous vaccine strains. Based on these studies, the WHO recommends the following antigens for the composition of the 2001-2002 campaign vaccines:

A / New Caledonia / 20 /99 (H1N1)-like strain
A / Moscow / 10 / 99 (H3N2)-like strain *
B / Sichuan / 379 / 99-like strain **
* Similar to A / Panama / 2007 / 99 (H3N2)
** Similar to B / Johannesbur / 5 / 99, B / Victoria / 504 /2000, or B / Guangdong / 120/ 2000

Flu vaccines may be used as from the age of 6 months. Recommended doses and models vary depending on age (0.25 ml. in children; 0.50 ml. in adults), and most of these are administered intramuscularly or deeply subcutaneously. According to the recommendations of the WHO and of the Centres for Disease Control and Prevention (CDC, Atlanta, USA), each dose is to contain at least 15 mgr of hemagglutinin for each of the strains recommended for the vaccine. Most adults have been in contact with the strains included in the vaccines and therefore one dose is sufficient, whereas children under 9 (who have never been vaccinated before) have probably never been exposed to the virus, which is why, for this age bracket, two doses with a four-week interval are recommended. Increasing the number of doses in people with a normal or altered immune system does not improve the response level, except for the children for whom it is indicated. One ought to be prudent in administering the vaccine to people who are hypersensitive to egg protein or to mercury thiosalicylate, or have a history of Guillain-Barré, although the control carried out through the USA side-effects surveillance system (Vaccine Adverse Event Reporting System, “VAERS”) for the periods 1992-93 and 1993-94 has detected an extremely low risk rate in connection with this side effect.

Immunogenicity
Traditional vaccines display good immunogenicity percentages in young adults, but lower percentages in elderly people. To avoid this problem, some studies with adjuvated vaccines have been undertaken, accepting the fact that in studies conducted on laboratory animals the adjuvants employed with different antigens substantially strengthen the humoral and T-helper cell response. The vaccine effectiveness depends on the age and immune system of the recipient, and on the relationship between the vaccine strains and those in circulation; certain studies published in the USA report a protective effectiveness against influenza (70-90% in healthy people under 65), a reduction in the hospitalisation rate (50-60%) and a prevention of mortality in elderly people (80%). Therefore, the positive effects of all flu vaccinations can be immediately observed through a reduction in complications (viral or bacterial pneumonia), a reduction in the hospitalisation rate, in mortality and in healthcare expenses in general (direct and indirect costs), thanks to the rapid post-vaccine appearance of a high number of hemagglutination inhibiting antibodies (neutralising similar strains). According to a number of authors, these positive effects displayed by all vaccines improve with the new adjuvant vaccines, which appear to show some advantages compared to the conventional ones, such as:
-A more rapid response in the appearance of immunity (2 - 3 weeks)
-A greater duration of immunity (8 - 12 months)
-An improved response, through the repetition of annual vaccination
-An improved response in people with an altered immune system
-A greater and more continuous response in elderly people

Vaccination Simultaneous to other Vaccines
The flu vaccine may be administered together with the antipneumococcic vaccine (in different inoculation areas), the former being administered once a year, and the latter at scheduled times. There are no special contraindications concerning administration together with other vaccines (with different syringes and needles in different areas). In children, it may be administered together with the Diphtheria-Tetanus-acellular Pertussis vaccine (DTPa) or with its whole-cell equivalent (DTP).

Safety of Influenza Vaccines
Flu vaccines contain dead viruses or fractions thereof and therefore cannot transmit the disease. These vaccines are entirely safe for all ages, and the most frequent side effects are those we have already mentioned: erythema in the inoculation area during the first two days and, less frequently, fever, tiredness or slight myalgia. The appearance of symptoms such as angioedema, asthmatic reactions or anaphylaxis is extremely rare. People who remember having suffered from lip or tongue edema or other symptoms after having eaten eggs are to report this before vaccination, since vaccines may contain minimum proteinic residues. The Guillen Barré Syndrome has rarely been associated with the flu vaccinations during the 1976, 1992-93 and 1993-94 campaigns but, in any case, its incidence is lower than that of post-influenza complications in non-vaccinated people. Although data on the administration of the flu vaccine to pregnant women are limited, the vaccine may be used in this period provided it is administered after the end of the first trimester.

Recommendations
In compliance with the recommendations of the CDC, the previously mentioned at-risk groups are vaccination candidates during the annual campaign, keeping in mind that on average antibodies appear after two weeks.
- In people over 65 it is definitely indicated, irrespective of the fact that they display risk factors in connection with the disease.
- Guests of old people’s homes, because during the epidemic periods the virus circulates freely.
- People suffering from chronic diseases, irrespective of their age.
- People displaying natural or drug-induced immunodepression problems, including HIV-positives (vaccination is indicated in patients affected by AIDS with normal or low CD4 count, even though a transitory replication of HIV may occasionally appear during the 2-4 weeks following vaccination).
- Children and teenagers in the 6 months – 18 years age bracket who have been subject to prolonged aspirin treatments, owing to the possibility of developing post-influenza Reye’s Syndrome - Pregnant women, if their second or third trimester of pregnancy coincides with the flue epidemic, and women during breast-feeding.
- People who, owing to their profession, are more likely to transmit the flu to people at risk: * Hospital or first-aid healthcare operators.
* Domiciliary healthcare staff (doctors or nurses who visit the patient at his/her home for diagnostic, treatment or therapy purposes)
* Voluntary workers
* Social workers
- Other groups for whom vaccination is recommended:
* Students living in residences or boarding schools
* People displaying risk factors and not vaccinated during the previous cam-paign, who travel in the southern hemisphere between April and September or in the Tropics at any time of the year. It is not necessary to re-vaccinate people who have been vaccinated during the campaign preceding the trip.
- In our opinion, in addition to the above-mentioned groups, the next campaigns will also have to work on the vaccination of children of all ages and of all people above 50, and, if resources will allow it, offer vaccination to the entire population.

Future Perspectives
Since the first inactive flu vaccination, prepared by Jonas Salk in 1940, up to the use of whole and fractioned vaccines and the current vaccine employing subunits, preparation, purification and preservation conditions have changed considerably, and we can state that they are useful and safe. Despite this, the flu vaccine continues to evolve and the virus yearly “antigenic slip” (related to minor mutations) involves the need for a new vaccination every year. In order to improve this situation, it would be important to search for a stable antigen for each strain. Finding a vaccine whose effects last for a few years represents a major goal, as well as obtaining virus cultures on cells, to avoid the need for a great supply of eggs. Within the guidelines that are currently followed to improve available vaccines, researchers are working on two aspects: the search for effective adjuvants that improve immunogenicity (humoral response with circulating neutralising antibodies and mucous response with the production of IgA) and the use of locally administered vaccines (inhalation or nose sprays with attenuated vaccines). Nor should we forget about the clinical trials against the flu virus with DNA or virosome vaccines. It will be necessary to better control the effectiveness and effect of the vaccination campaigns, through the assessment of a reduced hospitalisation rate, of the mortality rate and of healthcare expenses borne in connection with this disease, as well as working on alternatives in the event of epidemics, such as the resort to an adequate chemoprophylaxis, especially in people for whom the vaccine is contraindicated. We do not wish to close without acknowledging that fighting the flu represents a daily job, in that, as the WHO solicits, an “inter-pandemic control” is required, through the identification of “Significant Areas”, keeping in mind that:
-It is impossible to predict the appearance of a pandemic
-Health authorities are responsible for the surveillance This surveillance must be directed to two major fields:
* Serologic flu surveillance, since in this way it will be possible to quickly get to know the new strains, even if these do NOT cause pandemics (1976/USA/A H1N1; 1997/Hong Kong/ A H5N1; 1999 Hong Kong/ A H9N2)
* Development of prevention strategies:
- Increase in vaccination rates and in adequate epidemiological studies
- Increase in children vaccination
- Research of new vaccines
- Increase in the research for antiviral treatments
- Healthcare education
- Creation of National Strategy Planning Committees

With the development and implementation of these ideas we shall be better prepared to face the arrival of the next pandemic.

José Ramón de Juanes
Direttore del Servizio di Medicina Preventiva "Universidad Complutense" Madrid

(trad.Interpres-Giussano)