Charles John Palenik, MS, PhD, MBA
Infection Control Research and Services
Indiana University School of Dentistry
Indianapolis, Indiana
Phone: 317.274.5427
Email:

Influenza (the flu) is a contagious respiratory illness caused by 2 types of influenza viruses. Flu symptoms vary from mild to severe and can even lead to death. Although most healthy people recover from the flu without complications, some people, especially older ones (over 65 years), young children (6 months to 5 years), and any person with certain health conditions (such as congestive heart failure, asthma, or diabetes) or living in a long-term care facility, are at increased risk for serious complications. The best way to prevent the flu is to be immunized every year.^1,2

Each year in the United States 5% to 20% of the population gets the flu. More than 200,000 people require hospitalization, while about 36,000 people die from the flu. More than 90% of deaths occur in persons 65 years of age and older.^1,2

In the United States, the flu season usually runs from November through March. In some years, cases continue to develop in April and even May. For the last 25 years, the heaviest flu activity occurred in February.³

Virology

Influenza is a single-stranded, helically shaped, ribonucleic acid (RNA) virus, which comes in 3 types—A, B, and C—based on nucleic acid content. Type A influenza has subtypes that are determined by surface antigens—hemagglutinin (H) and neuraminidase (N). For influenza A viruses, there are 3 types of hemagglutinin in humans (H1, H2, and H3), which have a role in virus attachment to cells. Two types of neuraminidase (N1 and N2) have a role in virus penetration into cells.^1,2

Influenza A causes moderate to severe illness and affects all age groups. The virus affects humans and other animals. Influenza virus persists in nature primarily through wild birds, especially waterfowl. Most influenza A viruses are not pathogenic to humans and do not often mutate. Influenza B usually causes milder infection than type A and primarily affects children. Influenza C rarely causes human illness; however, many cases of infection could be subclinical. Type C has not been associated with epidemic disease.

The nomenclature to describe the type of influenza virus is expressed in a specific order—virus type, geographic site it was first isolated, strain number, year of isolation, and virus subtype (if any). An example would be A/Wisconsin/67/2005 (H3N2).

Having the Flu

The incubation period for influenza is usually 2 days, with a range between 1 and 4 days. The severity of the disease depends on the individual's immunologic experiences with similar influenza viruses. In general, only about 50% of infected persons will develop classic influenza symptoms.^1-3

Symptoms of flu include an abrupt onset of fever (often 101°F to 102°F), headache, extreme fatigue (to the point of prostration), dry cough, sore throat, runny/stuffy nose, muscle aches, and stomach distress, including nausea, vomiting, and diarrhea. Gastrointestinal problems are more common in children than in adults. Sometimes it is difficult to differentiate between the flu and the common cold (Table 1).^1,2,4

With the flu, the onset of disease is so abrupt that many people can recall the exact hour fever developed. Other symptoms are somewhat common to both diseases. These include sneezing, a runny/stuffy nose, chest discomfort, and cough. Systemic symptoms and fever usually last from 2 to 3 days, but rarely more than 5 days. Recovery is usually rapid. However, some people experience a lingering depression and a lack of energy or strength, which may last several weeks.

The most common complications of a cold are sinus congestion and earache, while the flu can lead to bronchitis and pneumonia, which can be life threatening. A recent study reported that peak deaths from heart disease seem to coincide with peak influenza activity.⁴ This suggests that influenza contributes to heart attacks. It appears that severe influenza can destabilize atherosclerotic plaques in coronary arteries, which can lead to acute myocardial infarctions. Controlling influenza infections, especially in the elderly, could prevent an estimated 90,000 coronary deaths each year in the United States.^1-5

Spread of Influenza

Influenza spreads from person to person by coughing or sneezing (respiratory droplets) and to a lesser extent through unclean hands (touching something contaminated and then touching your eyes, nose, or mouth). Virus-laden droplets are large (>5 mm) and do not readily travel more than 3 ft. The fewer the number of items infected people contact and the less they move about, the greater the chances for reducing disease transmission. Disease reduction also includes the use of respiratory hygiene/cough etiquette.^1-6 Table 2 lists the US Centers for Disease Control and Prevention's (CDC) recommendations to contain respiratory secretions.^7,8 Healthy adults may be able to spread infection 1 day before the onset of symptoms and up to 5 days after becoming ill. Children can transmit influenza for 10 or more days.

Influenza Vaccination

The best way to prevent influenza is to be vaccinated each year. October and November are the best times for vaccination. However, getting vaccinated in December or even later can still be beneficial because most influenza outbreaks occur in January and February. It takes about 2 weeks for sufficient numbers of protective antibodies to form. Therefore, earlier vaccination should translate into longer protection. Good health habits also can be protective (Table 3).⁹ Some people are at increased risk, so they especially need vaccination.¹⁰

• People at high risk for complications from the flu:
• Children aged 6 months to 5 years
• Pregnant women
• People 50 years of age or older
• People of any age with certain medical conditions
• People living in nursing homes or other types of long-term care facilities
• People who live with or care for those at high risk for complications from flu:
• Household contacts of people at high risk
• Household contacts and out-of-home caregivers of children younger than 6 months of age
• Health care workers

Influenza Vaccines

There are 2 types of influenza vaccines. The "flu shot" is a single injection of inactivated vaccine (killed viruses). Any individuals older than 6 months, including healthy people and those with chronic conditions, can receive this injection.^1-5,10 In 2003, the US Food and Drug Administration approved a live attenuated influenza vaccine (LAIV) for use in the United States. Administration of LAIV is by an intranasal route and contains the same 3 viruses as the injected vaccine. When sprayed into the nose, the viruses stimulate an immune response. LAIV is for use in healthy people aged 2 to 49 years, excluding pregnant women.

The influenza viruses present in a given region vary from year to year. This means that having the flu does not provide lifelong protection against future infections. Immunization against one influenza virus does not guarantee protection against new strains of influenza that may be circulating. The composition of both types of vaccine includes 2 strains of influenza A and 1 type of influenza B. Each year the vaccines' specific composition is based on the viruses predicted to be present in the coming flu season. When the match between the vaccine and the circulating viruses is correct, influenza vaccines can reduce infection in 70% to 90% of healthy adults under the age of 65 years.

Health care workers need vaccination against influenza each year. The CDC strongly encourages employers to provide vaccine for their workers. Some facilities also offer vaccine to their employees' family members. A properly vaccinated employee is less likely to miss work because of influenza and less likely to infect patients or family members (and thus not miss more work). Providing vaccination is cost-effective for employers.

Vaccination rates of health care workers, however, hover at about 40%. It is probably best that health care workers receive the influenza injection because most health care workers can receive it and because there is a slight possibility that the virus could move from an LAIV recipient to another person, especially if that person is immune-suppressed.^1-5,10,11 Some health care workers should consult a physician before seeking vaccination, including those who^1,3:

• have a severe allergy to chicken eggs.
• have had a severe reaction to an influenza vaccination in the past.
• developed Guillain-Barré syndrome within 6 weeks of getting an influenza vaccine previously.
• have a moderate or severe illness with a fever. These workers should wait to get vaccinated until their symptoms lessen.

Conclusion

Dental workers are at increased risk for occupational acquisition of influenza. Infection can affect your workplace performance and jeopardize the health of patients, coworkers, and family members. Annual vaccination and use of effective infection measures and equipment are essential to prevent the spread of influenza.

References

1. US Centers for Disease Control and Prevention. Prevention and control of influenza. Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2007. MMWR Recomm Rep [serial online]. 2006;56 (RR-6):1-60. Available at: www.cdc.gov/mmwr/PDF/rr/rr5606.pdf. Accessed Aug 24, 2007.
2. Public Health Foundation. Influenza. In: Epidemiology and Prevention of Vaccine-Preventable Disease (The Pink Book). 10th ed. Waldorf, Md: Public Health Foundation; 2007:235-256. Available at www.cdc.gov/vaccines/pubs/pinkbook/downloads/flu.pdf. Accessed Aug 14, 2007.
3. US Centers for Disease Control and Prevention. Key facts about influenza and the influenza vaccine. August 2006. Available at: www.cdc.gov/flu/keyfacts.htm. Accessed Aug 14, 2007.
4. Madjid M, Miller CC, Zarubaev VV, et al. Influenza epidemics and acute respiratory disease activity are associated with a surge in autopsy-confirmed coronary heart disease death: results from 8 years of autopsies in 34,892 subjects. Eur Heart J. 1007;28:12051210.
5. US Centers for Disease Control and Prevention. Influenza: the disease. March 2007. Available at: www.cdc.gov/flu/about/disease.htm. Accessed Aug 14, 2007.
6. US Centers for Disease Control and Prevention. Infection control guidance for the prevention and control of influenza in acute-care facilities. February 2007. Available at: www.cdc.gov/flu/professionals/infectioncontrol/healthcarefacilities.htm. Accessed Aug 14, 2007.
7. US Centers for Disease Control and Prevention. Control of influenza outbreaks in institutions. August 2006. Available at: www.cdc.gov/flu/professionals/infectioncontrol/institutions.htm. Accessed Aug 14, 2007.
8. US Centers for Disease Control and Prevention. Cough. Cover your cough. Available at: www.cdc.gov/flu/protect/covercough.htm. Accessed Aug 24, 2007.
9. US Centers for Disease Control and Prevention. Good health habits for preventing the flu. October 2006. Available at: www.cdc.gov/flu/protect/habits.htm. Accessed Aug 14, 2007.
10. US Centers for Disease Control and Prevention. Key facts about influenza (flu) vaccine. October 2006. Available at: www.cdc.gov/flu/protect/keyfacts.htm. Accessed Aug 14, 2007.
11. US Centers for Disease Control and Prevention. Recommendations for influenza vaccination of health-care workers. September 2006. Available at: www.cdc.gov/flu/professionals/vaccination/hcw.htm. Accessed Aug 14, 2007.