History and exam

Key diagnostic factors

Seasonal influenza tends to have a seasonal outbreak pattern, with epidemics usually occurring between late autumn and early spring.

Suspicion for seasonal influenza should be high if there is a documented outbreak in the community. During the influenza season, the CDC publishes weekly updates online that summarize information about influenza activity. CDC: FluView - weekly influenza surveillance report external link opens in a new window

Patients should be asked whether they receive the seasonal influenza vaccine every year. Healthy adults vaccinated with intramuscular inactivated vaccine have a reduced probability of influenza A or B infection and influenza-like illness, although the absolute effect may be modest.[40][57][58] Vaccination in healthy children (with live attenuated vaccine or inactivated vaccine) can reduce influenza and influenza-like illness; the effect varies across populations studied.[42]

Studies in older patients have shown the presence of an acute onset of fever and cough to have a positive predictive value of only 30% to 53% for seasonal influenza in nonhospitalized and hospitalized patients, respectively.[87][88]

A study of vaccinated older people with chronic lung disease reported that cough was not predictive of laboratory-confirmed influenza virus infection, although having both fever or feverishness and myalgia had a positive predictive value of 41%.[89]

Young children are less likely to report typical influenza symptoms such as fever and cough.[90]

If there is an exacerbation of fever and cough with purulent sputum and dyspnea, a secondary bacterial pneumonia should be suspected. A chest x-ray confirms infiltrates.

Secondary bacterial pneumonia is an important complication of influenza and contributes to approximately 25% of all influenza-associated deaths.[91]

Other diagnostic factors

Oropharyngeal symptoms other than sore throat with associated hyperemia are not common.

A nonspecific finding that may be more common in children.[86]

An uncommon symptom that should prompt an evaluation for a complication of influenza such as bacterial pneumonia, particularly when associated with fever, cough, and purulent sputum.[91]

Risk factors

Compared with young, healthy adults, people aged ≥65 years are at greater risk of serious complications from influenza, and are more likely to have comorbid conditions that may be exacerbated by influenza infection. It is estimated that 90% of seasonal influenza-related deaths and more than 60% of seasonal influenza-related hospitalizations in the US each year occur in people ≥65 years.[11][12] Influenza can be a very serious disease when immune defenses become weaker with age. This age also brings a greater likelihood of comorbid conditions that may be exacerbated with influenza infection.[13]

Although children with chronic medical conditions such as pulmonary, renal, or cardiac disease have a high risk of complications of influenza, otherwise healthy children are at risk simply because of their age. Children aged <5 years are more likely to be hospitalized than older children; those aged <2 years are at elevated risk of complications attributable to influenza.[14]

In patients with moderate or severe COPD, the presence of any virus in upper airway secretions is strongly associated with the development of COPD exacerbations. These data support the causative role of viruses in triggering COPD exacerbations in the community.[15][16]

In older populations, vaccination against seasonal influenza is associated with reductions in the risk of hospitalization for heart disease, cerebrovascular disease, and pneumonia or influenza, as well as the risk of death from all causes during influenza seasons. These findings highlight the benefits of vaccination and support efforts to increase the rates of vaccination among older people.[16][17][18] [ Cochrane Clinical Answers logo ] [ Cochrane Clinical Answers logo ]

People with diabetes are at greater risk of complications due to their underlying disease.[19] Diabetes confers a 5% to 12% increase in mortality from seasonal influenza infection, thought to be due to increased risk of metabolic disruption, ketoacidosis, impaired immune response, and increased carrier rates of staphylococci and streptococci.[20]

Hemoglobinopathies such as sickle cell disease involve abnormalities not just in red blood cells but also in vascular endothelium, white blood cell function, coagulation, and inflammatory response. Routine influenza vaccination is recommended for infection prevention.[21]

Infection is the leading cause of morbidity and mortality in immunocompromised patients such as hematopoietic/solid organ transplant recipients and individuals with HIV.[22][23][24] Inactivated seasonal influenza virus vaccine is preferred over live virus vaccine for household members, healthcare workers, and others coming in close contact with severely immunosuppressed people requiring care in a protected environment.[25] In one study, vaccination with a high-dose trivalent vaccine resulted in higher levels of seroprotection in people with HIV.[26] Trivalent inactivated influenza vaccine is also immunogenic in pregnant women with HIV.[27]Inactivated vaccine should be used with caution in severely immunocompromised patients (e.g., patients receiving chemotherapy, radiation therapy, or other immunosuppressive therapy, including high-dose corticosteroids), as there may be a reduced response to vaccination. However, adjuvanted vaccine has been shown to be safe and immunogenic in the transplant population.[28] Intranasal live-attenuated vaccine is contraindicated in immunosuppressed or immunocompromised patients.

Patients with CKD are at increased risk of influenza complications.[13] Influenza vaccine is currently recommended for patients with CKD by the Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention.[29] In observational studies, influenza vaccination is associated with decreased risk of influenza-related hospitalizations, deaths, and physician visits.[30]

Immune, respiratory, and cardiovascular changes make pregnant women more prone to severe illness from influenza.[31] Pregnant women with influenza have a greater risk of preterm labor and delivery.[32] Trivalent inactivated influenza vaccine is immunogenic in both HIV-infected pregnant women and pregnant women who are not infected.[27] With regard to safety, maternal influenza vaccination does not increase risk of congenital malformations.[33][34][35]

The goal is to prevent transmission of the virus to a high-risk population.

Inactivated influenza virus vaccine is preferred over live virus vaccine for household members, healthcare workers, and others coming in close contact with severely immunosuppressed people requiring care in a protected environment.

Healthcare workers play an important role in protecting public health. The Advisory Committee on Immunization Practices of the Centers for Disease Control and Prevention recommends that all healthcare workers receive an annual influenza vaccination to limit the spread of infection.[36] Inactivated influenza virus vaccine is preferred over live virus vaccine for household members, healthcare workers, and others coming in close contact with severely immunosuppressed people requiring care in a protected environment.

As a vaccinated healthcare worker, there is protection for family at home as well as patients at work from possible influenza transmission. Influenza outbreaks in hospitals and long-term care facilities have been attributed to low vaccination rates among healthcare professionals.[37]

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