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Virology

Influenza Season 2012

Influenza Season 2012

Influenza viruses are RNA viruses with segmented genomes that have the important characteristic of mutation. These are either minor drift mutations or significant shift mutations that create viruses to which human populations have varying degrees of immunological memory. In some instances, new viruses are created by mutational changes or reassortment with influenza viruses that circulate in animals, (e.g. pigs, ducks) to which human populations are immunologically naïve. These may lead to global pandemics, sometimes with significant mortality.

Influenza A and B viruses cause seasonal epidemics every winter. New and different A and B influenza viruses emerge every year and if very different to previous circulating viruses, they may cause pandemics. Influenza C infections may cause mild respiratory illness, but these infections are generally not associated with epidemics. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: the haemagglutinin (H) and the neuraminidase (N). There are 17 haemagglutinin subtypes and 10 neuraminidase subtypes. Influenza A viruses can be further classified into strains. In 2009, a new influenza A (H1N1) virus emerged, which was quite different from and replaced the previously circulating human influenza A (H1N1) virus, which was first isolated about 40 years ago. Influenza B viruses are not divided into subtypes, but can be further classified into different strains.

Disease activity

The 2012 influenza season in South Africa started a few weeks ago with sporadic cases of mainly influenza B and more recently, influenza A H3N2 primarily. There has been a very low level of influenza A H1N1 (swine flu) activity so far. The influenza season in South Africa typically starts in the first week of June, but may start as early as April or as late as the middle of July.

The graph that follows represents surveillance reports submitted to the GISRS-FluNet system and posted on the World Health Organisation (WHO) website in real time, providing the latest global information on circulating influenza1. GISRS is a global public health laboratory network coordinated by WHO, currently consisting of globally distributed National Influenza Centres (NICs) in 107 member states, 6 WHO Collaborating Centres for Influenza (CCs), 4 WHO Essential Regulatory Laboratories (ERLs) and 12 WHO H5 Reference Laboratories.

Graph 1: Number of positive specimens for influenza by subtype in South Africa 2011-2012 as on 29 June 2012

Influenza

Northern hemisphere

The seasonal peak for influenza has passed in most countries in the northern hemisphere. Different viruses predominated in the northern hemisphere during the 2011-12 influenza season, with influenza B showing a predominance in North America, Canada, Northern China and Mongolia over influenza A (H3N2). In the United States, the Republic of Korea and Japan the proportions were reversed and A (H3N2) was more common. In Mexico, influenza A(H1N1) was responsible for the entire influenza season. In Europe, most circulating influenza viruses were A(H3N2) with only very small numbers of A(H1N1) and influenza B circulating.

At the beginning of the influenza season in the northern hemisphere, most viruses tested were antigenically closely related to those found in the current trivalent seasonal vaccine. However, by midseason, divergence was noted in both the United States and Europe in the A(H3N2) viruses tested and more recently significantly reduced cross reactivity with the vaccine viruses.

Influenza B virus detections have been both from the Victoria and Yamagata lineages with the former slightly more common in China and parts of Europe.

Polymerase chain reaction (PCR) on respiratory/throat or naso-pharyngeal specimens within 2-3 days after onset of symptoms is the laboratory method of choice for the diagnosis of influenza.

Prevention and vaccination

Trivalent inactivated influenza vaccine takes 10-14 days before it can provide protective antibodies. It is not too late to be vaccinated for the 2012 season. Annual vaccination against influenza is recommended for any adult who wants to reduce the risk of becoming ill with influenza or of transmitting it to others.

The NIC D recommends vaccination for:2
Adults or children at high risk for influenza and its complications
because of underlying medical conditions for which they are
receiving regular medical care, including chronic pulmonary
or cardiac disease, chronic renal disease, diabetes mellitus
and similar metabolic disorders, individuals who are immunesuppressed
(including HIV-infected persons with CD4 counts
>100 cells/μl), and morbidly obese (BMI≥40) individuals.
Pregnant women, irrespective of stage of pregnancy.
Residents of old-age homes, chronic care and rehabilitation institutions.
Children on long-term aspirin therapy to prevent Reyes syndrome.
Medical and nursing staff in contact with high-risk persons.
Adults and children who are in reguarl contact with high-risk
family members.
All those older than 65.

Anyone who wants to protect themselves from the risk of contracting influenza, especially in settings where large-scale absenteeism could cause significant economic losses.

Nosocomial and institutional (old age homes/schools/ universities/daycare) spread of influenza can be significantly decreased by workers and students being vaccinated. This strategy should be considered as the primary approach for infection control. Individuals should be vaccinated every year, because of the variability of the circulating influenza viruses. The second most important way to prevent the spread of influenza in these situations is hand washing and quick disposal of used tissues.

The vaccine formulation for the current South African 2011/2012 influenza season is:

A/California/7/2009 (H1N1) pdm09-like virus A/Perth/16/2009 (H3N2)-like virus B/Brisbane/60/2008-like virus3

For those traveling to the northern hemisphere, they need influenza vaccine with the 2012/2013 formulation to be protected from the predicted circulating viruses for those countries winter 2012. The WHO recommendation for this vaccine is:

A/California/7/2009 (H1N1) pdm09-like virus A/Victoria/361/2011 (H3N2)-like virus B/Wisconsin/1/2010-like virus

Vaccine side effects

Influenza vaccine cannot cause influenza as it is an inactivated viral vaccines. This common misconception has probably arisen because multiple types of viruses cause the common cold (e.g. adenovirus, rhinovirus and coronavirus groups), which also circulate during the winter months. Usually the common cold is less
severe and of shorter duration than "flu".

Reye's syndrome

This is primarily a children's disease, but can occur at any age. It affects all organs of the body and is most harmful to the brain and the liver, causing raised intracranial pressure and massive accumulations of fat in the liver.

The cause of Reye's syndrome is unknown, but aspirin or salicylate-containing medications to treat viral illnesses increases the risk of developing this condition. It may be fatal and successful management depends on early diagnosis and is aimed at reducing brain swelling, reversing metabolic injury and preventing complications in other organs. Recovery depends on the severity of the brain swelling and may be complete, but in some cases varying degrees of brain damage occur.

Influenza treatment

The use of antivirals like oseltamivir (Tamiflu®) for prevention should be limited to high-risk situations and should not be used indiscriminately. Oseltamivir must be started within 24-48 hours after the onset of symptoms and should be continued for five days. It is available off-label for premature neonates (dose 1 mg/ kg 12 hourly) and infants 0-12 months (dose 3 mg/kg 12 hourly). For children and adults over 40kg the dose is 75mg 12 hourly. Zanamivir (Relenza®) is also used in the treatment of influenza A and B in adults and children over the age of 12 years. It is administered using oral inhalations with a diskhaler (two inhalations, 10 mg, 12 hourly for five days) and must be started within 36 hours of onset of symptoms.

Sources

  1. World Health Organisation GISRS-FluNet.
  2. Communicable Disease Communiqué. National Institute of Communicable Diseases. March 2012.
  3. South African Medicines Formulary. University of Cape Town. 10th Edition. 2012.

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