INFLUENZA is one of several diseases causing fever and respiratory symptoms that might raise suspicions of SARS. Its resemblance to SARS (Severe Acute Respiratory Syndrome) has raised concerns as a recurrence of SARS during the influenza season cannot be ruled out.
However, influenza itself is of particular concern because of the potential for institutional and community outbreaks and regional epidemics. Influenza typically infects 10% to 20% of the total population during seasonal epidemics, resulting in three to five million cases of severe illness and at least 250,000 to 500,000 deaths each year worldwide.
Influenza is caused by a virus that attacks mainly the upper respiratory tract – the nose, throat and bronchi, and, rarely, the lungs. The infection usually lasts for about a week. It is characterised by sudden onset of high fever, myalgia (muscle pain), headache and severe malaise (vague discomfort), non-productive cough, sore throat, and rhinitis (nasal inflammation).
While most people recover within one to two weeks without requiring any medical treatment, for certain groups like the very young, the elderly and people suffering from medical conditions such as lung diseases, diabetes, cancer, kidney or heart problems, influenza poses a serious risk. In these people, the infection may lead to severe complications, and death is a distinct possibility.
Influenza viruses that cause human disease are divided into two groups: A and B. Influenza A has 2 subtypes which are important to humans: A(H3N2) and A(H1N1), of which the former is currently associated with most deaths.
Influenza viruses are defined by two different protein components, known as antigens, on the surface of the virus. They are spike-like features called haemagglutinin (H) and neuraminidase(N) components.
The genetic makeup of influenza viruses allows frequent minor genetic changes, known as antigenic drift, and these changes require annual reformulation of influenza vaccines.
Sometimes their genetic makeup may undergo major changes, known as antigenic shift. The resultant new strain of virus is of particular danger to a human population as no immunity has been developed against the virus. If the virus spreads fast enough, it may cause a full-blown epidemic.
The Fujian flu that recently swept over Europe and the US is a slightly mutated form of A/Panama/2007/99, a common strain of influenza virus.
How it spreads
Airborne transmission of influenza viruses occurs particularly in crowded enclosed spaces. Transmission also occurs by direct contact with droplets disseminated by coughs and sneezes, and contamination of the hands.
All countries worldwide are affected. In temperate regions, influenza is a seasonal disease occurring in winter, affecting the northern hemisphere from November to March and the southern hemisphere from April to September.
In tropical areas like ours, there is no clear seasonal pattern, and influenza may occur at any time of the year. There has been no significant increase in influenza-like respiratory illnesses of late here, according to Dr Ashoka Menon, a consultant chest physician in Klang Valley.
“While there has been some increase in cases of Respiratory Syncytial Virus (RSV) infection in children recently, this may be partly due to the increase in requests for the diagnostic tests as a result of greater awareness,” added Prof Adeeba Kamarulzaman, a consultant infectious disease physician.
Who is at risk?
All individuals are at risk in any country during the influenza season with the elderly, people with pre-existing chronic diseases and young children most susceptible.
How do I know if I have been infected?
Respiratory illness caused by influenza is difficult to distinguish from illness caused by other respiratory pathogens based on the symptoms alone. During influenza outbreaks, the majority of persons seeking medical advice for upper respiratory tract infections are likely to be infected by influenza.
Rapid diagnostic tests have recently become available that can be used to detect influenza viruses within 30 minutes. “These tests employ immunological and molecular techniques such as immunofluorescence (IF) assays, enzyme immunoassays (EIA), and polymerase chain reaction (PCR). At tertiary care hospitals like University Malaya Medical Centre (UMMC), IF assays on throat swabs or nasopharyngeal aspirates can be performed to confirm the diagnosis,” Prof Adeeba explained.
“The diagnosis may also be established retrospectively by serological methods, primarily haemagglutination-inhibition (HI). A four-fold or greater rise in antibody titres demonstrated between serum specimens obtained during the acute and the convalescent phase of the illness (specimens obtained 10 to 14 days later) is considered diagnostic,” she added.
Influenza viruses evolve rapidly, changing their antigenic characteristics, so that vaccines need to be modified each year to be effective against currently circulating influenza strains.
The recommended vaccines to be used in the 2003-2004 season (northern hemisphere winter) contain an A/New Caledonia/20/99(H1N1)-like virus, an A/Moscow/10/99(H3N2)-like virus and an A B/Hong Kong/330/2001-like virus.
Recent virus isolates had been shown to be distinguishable from A/Panama/2007/99 and similar to A/Fujian/411/2002. However since there is no A/Fujian/411/2002-like virus isolated in embryonated eggs that makes it suitable as a vaccine candidate, and since many recent isolates are closely related to A/Panama/2007/99, the WHO recommended that the A(H3N2) component of vaccines to be used in the 2003-2004 season contain an A/Moscow/10/99 (H3N2)-like virus.
The recent epidemics in other parts of the world have raised concerns among some patients. Both Prof Adeeba and Dr Ashoka acknowledged that there were people, particularly travellers, who came to see them enquiring about the disease and its vaccine.
Dr Ashoka added that even though the currently available vaccine in our country does not contain the antigenic component of the Fujian strain, it does afford some protection against it.
“The protective efficacy of the vaccine is largely determined by the relationship (closeness of 'fit') between the strains in the vaccine and viruses that circulate in the outbreak,” explained Prof Adeeba. “If this 'fit' is close, rates of protection of 50 to 80% against clinical influenza would be expected. In large studies, the protective efficacy achieved is 88-90%.”
A vaccine works by inducing the body's immune system to produce antibodies against the disease-causing agent, in this case the influenza viruses. Usually, the vaccine contains certain parts of the viruses that will not cause the disease when introduced into the body but is capable of triggering off an immune response to produce sufficient antibodies. Antibodies are the soldiers in our immune system responsible for warding off infections. Sometimes a vaccine is made up of a live but weakened form of the virus.
Who should get the vaccine?
The flu vaccine is basically recommended for people in high-risk groups like individuals above the age of 65, those with chronic illnesses like kidney failure requiring dialysis, heart or lung conditions, people who have weakened immune system and health care workers, according to Prof Adeeba.
“Unlike in temperate regions, there is not a distinct 'flu season' in our population. Therefore the timing of vaccination is always not so clear cut,” she added.
“Maintain good hygiene!” Prof Adeeba stressed when asked about advice for the public to avoid this contagious disease.
For travellers in the highest risk groups for severe and complicated influenza who have not been or cannot be vaccinated, the prophylactic use of antiviral drugs such as zanamivir and oseltamivir is indicated where they are available. Amantidine and rimantidine may also be considered.
What else can be done?
Whenever possible, avoid crowded enclosed spaces and close contact with people suffering from acute respiratory infections.
Similarities to SARS
“At present SARS still appears to be confined to countries with previous confirmed cases. Unless the person has a history of travel to those places or has been in contact with someone who has been in those places, the probability of them having SARS is reasonably small,” Prof Adeeba explained.
“On the other hand common flu occurs sporadically here in Malaysia,” she added. “The chances of an ordinary Malaysian who stays within the country being infected with the epidemic flu are reasonably low.”
Some historical perspectives
Three times in the last century, the influenza A viruses have undergone major genetic changes, mainly in their H-component, resulting in global pandemics and large tolls in terms of both disease and deaths. The most infamous pandemic was the “Spanish flu” which affected large parts of the world population and is thought to have killed at least 40 million people in 1918-1919.
Despite being known as the Spanish flu, it actually came to Europe from America. As Spain had no censorship regulations then, the outbreak was widely publicised in their press and thus the name “Spanish flu” came about.
More recently, two other influenza A pandemics occurred in 1957 (“Asian influenza”) and 1968 (“Hong Kong influenza”) and caused significant morbidity and mortality globally.
In contrast to the current influenza epidemics, these pandemics were associated with severe outcomes, even among the younger healthy individuals. The Spanish flu was the most notorious of all as the death rates were highest among young healthy adults.
1. WHO Fact Sheet On Influenza www.who.int/mediacentre/factsheets/2003/fs211/en
2. Influenza Vaccination For The 2003-2004 Season: WHO Recommendations In the Context Of Concern About SARS
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