Influenza Vaccination: 2010 Update

Introduction

Influenza (also referred to as the flu) is a contagious respiratory illness caused by a set of viruses; it causes annual outbreaks and epidemics of disease worldwide. The influenza viruses are separated into categories based on antigenic properties. The primary categories are influenza A and influenza B. Influenza A viruses are differentiated based on the presence of the surface antigens hemagglutinin (H) and neuraminidase (N). The influenza B viruses are not further differentiated, although there are 2 distinct genetic lineages.¹

Seasonal strains of influenza are those that are common each year. Examples include the influenza A (H3N2) and influenza B viruses. The yearly influenza vaccination typically protects against these strains, but they still have the capability of causing epidemics. New influenza strains, also called novel strains, arise when there is antigenic drift caused by mutations and recombination during viral replication. These strains are potentially dangerous, as they can cause pandemics (ie, person-to-person transmission on multiple continents) when there is little or no existing immunity. This was the case in 2009 with the appearance of a novel H1N1 virus with antigens thought to have evolved from a previous novel H1N1 strain which was associated with a pandemic in 1918.¹

The latest Advisory Committee on Immunization Practices’ (ACIP) guidelines include several important changes and updates (Table).¹ Routine influenza vaccination is now recommended for all persons 6 months of age or older. As previously recommended, all children aged 6 months to 8 years who receive a seasonal influenza vaccine for the first time should receive 2 doses. Children who received only 1 dose of a seasonal influenza vaccine in the first influenza season should receive 2 doses, rather than 1, in the following influenza season.

Influenza: Morbidity and Mortality

Uncomplicated influenza infection is characterized by the presence of upper respiratory symptoms combined with the abrupt onset of fever, headache, soreness, and fatigue. These symptoms are generally self-limiting and resolve in 2 to 5 days, although they may last for more than a week.

Complications of influenza are the primary cause of morbidity and death. The major complication is bacterial pneumonia, which contributes to approximately 25% of all influenzarelated deaths.² Viral pneumonia, while less common, is also associated with a very high mortality rate. Other complications include rhabdomyolysis, central nervous system involvement, and cardiac and pulmonary exacerbations.

In the United States, more than 200,000 people are hospitalized and an estimated 3,300-49,000 die each year from seasonal influenza and its complications. The vast majority of these cases occur in persons aged 65 years and older and children aged 24 months and younger.³

It is important to note, however, that the novel 2009 H1N1 virus resulted in higher mortality in persons less than 65 years of age with confirmed disease when compared with seasonal influenza strain. It is believed that the elderly population had partial immunity to this strain due to exposure to an antigenically similar influenza A strain that circulated earlier in the 20th century.¹

Prevention vs. Treatment

The best way to prevent influenza infection is through seasonal, yearly vaccination with additional vaccinations against novel strains when necessary. Direct treatment options for influenza infection in the United States are focused on 4 antiviral agents.⁴ Zanamivir and oseltamivir phosphate are approved for the treatment of uncomplicated illness due to influenza A and B and for preventative use. Two older agents, amantadine and rimantadine, are approved for the treatment and prevention of influenza A, but many strains have developed resistance to these medications. When administered within the first 24 to 48 hours of infection, these antiviral medications have been shown to reduce the severity and duration of infection by a few days.¹ The US Food and Drug Administration (FDA) cautions that these antiviral agents are not a substitute for vaccine and should only be used as an adjunct to vaccine in the control of influenza.⁴

Efficacy of Vaccination

The efficacy of vaccination is directly related to 2 factors: strain match and age of the vaccinated patient. When the vaccine virus strains are a close match to those circulating in the general population, vaccination will lower the risk of infection by as much as 90% in healthy individuals under the age of 65 years. While the rate of protection is lower in the elderly, vaccination has been estimated to reduce the incidence of hospitalization and death from influenza in these patients by as much as 70%.⁵

Yearly flu vaccination should begin as soon as the vaccine becomes available, generally in September, and continue through the following May. In one study, immunity from vaccination was as high as 100% within the first year and 68% in the second year.¹ Other trials have demonstrated efficacy extending into the third year after vaccination.¹ Since the composition of the seasonal flu vaccine changes from year to year, it is important to get vaccinated annually to provide optimal immunity against seasonal and novel strains.

2010 Seasonal Vaccination

The 2010 seasonal influenza vaccination will be trivalent and provide coverage against the following strains: the novel 2009 H1N1 virus (A/California/7/2009), an H3N2 virus (A/Perth/16/2009), and an influenza B virus (B/Brisbane/60/2008).¹

It will be broadly separated into 2 administration options: injection (also known as trivalent inactivated vaccine [TIV]) or nasal spray (also called live attenuated influenza vaccine [LAIV]). The “flu shot” contains viruses that have been killed and have no capability of causing influenza. The nasal vaccine contains live attenuated viruses that have been weakened and are unable to effectively replicate in the lungs, but have the potential to cause mild signs or symptoms related to infection.

In some influenza seasons, the supply of the inactivated injection has been a cause for concern. The availability of several formulations should help alleviate shortages in the 2010 season. The FDA has licensed at least 4 drug manufacturers to provide the injectable flu vaccine. A recently approved high-dose formulation targeted for patients over the age of 65 years will be available this year;¹ however, it is important to remember that several standard-dose TIV injections are approved for use in this patient population and provide adequate coverage. Also, 2 TIV formulations have received expanded age indications to provide broader coverage among pediatric patients.^1,6,7

Multidose TIV vials contain the preservative thimerosal. Preservativefree, single-dose preparations are also available. The intramuscular route is recommended. The medication should be dosed in accordance with individual manufacturer package inserts. LAIV is supplied as a single-use sprayer. Approximately half (0.1 mL) of the contents should be sprayed into one nostril with the head held in the upright position. The remainder should be administered in the other nostril.¹ All vaccine formulations should be stored at 35^oF to 46^oF and never frozen.

Screening and Adverse Reactions

High-risk populations include children younger than 5 years (especially children younger than 2 years), adults older than 65 years, pregnant women, and people with certain existing medical conditions such as: asthma, neurological conditions, chronic lung disease, heart disease, blood disorders, endocrine disorders (eg, diabetes), kidney disease, liver disease, metabolic disorders, weakened immune system (due to disease or medication), morbid obesity, and patients under the age of 19 years on chronic aspirin therapy. Of note, Native Americans and Alaskan natives were especially vulnerable in the 2009 flu season. Finally, people who work or live in assisted care facilities, health care facilities, or are household contacts for such people, are at high risk for influenza-related complications.⁸ It is imperative for these people to receive the influenza vaccination as soon as possible.

Both vaccinations are generally well-tolerated with serious reactions rarely occurring. For TIV, patients need to be screened for a history of anaphylactic egg allergy (due to how the vaccine is prepared), moderate to severe febrile illness (defer immunization until it resolves), and a history of Guillain- Barré syndrome. Otherwise, the primary adverse events are local injectionsite reactions and low-grade fever, or other mild systemic symptoms, which occur for less than 24 hours. For LAIV, patients should be healthy and between the ages of 2 years and 49 years. The primary side effects are runny nose, headache, and congestion.³

Pharmacists should remember that it is a federal law for all persons who receive vaccination to be provided with current Vaccine Information Statements, which are available on the Centers for Disease Control and Prevention’s Web site.

Summary

According to the latest Advisory Committee on Immunization Practices’ guidelines, everyone aged 6 months and older should receive an annual, seasonal influenza vaccination. Additionally, high-risk patients should continue to be a focus of health care providers, especially when vaccine supply is limited. The 2010 seasonal flu vaccine will be trivalent and provide coverage against the novel 2009 H1N1 pandemic virus, as well as another influenza A virus, and an influenza B virus. The inactivated injectable flu vaccine is safe and efficacious in all patients for whom they are indicated. Several formulations are available for the 2010 season to help alleviate shortages. Individual manufacturer recommendations should be consulted and followed. The use of intranasal vaccination should be restricted to healthy individuals aged 2 years to 49 years. PT

Dr. Copeland is a clinical pharmacist specialist at Parkwest Medical Center in Knoxville, Tennessee. He has been certified by APhA in Pharmacist-Based Immunization Delivery since 1998 and gives several hundred vaccinations annually.

This article was supported by Novartis Vaccines.