What are the effects of antiviral treatment of influenza in adults?

LIKELY TO BE BENEFICIAL

Oral Amantadine for Early Treatment of Influenza A in Adults (Duration of Symptoms Reduced)

One systematic review and three additional randomized controlled trials (RCTs) have found that oral amantadine reduces the duration of influenza A symptoms by about one day compared with placebo. We found insufficient evidence about adverse effects in this setting. We found no good evidence of benefit if amantadine is started more than two days after symptom onset.

Orally Inhaled Zanamivir for Early Treatment of Influenza A and B in Adults (Duration of Symptoms Reduced)

One systematic review has found that orally inhaled zanamivir reduces the duration of influenza symptoms by about one day compared with placebo. Adverse effects were similar in people taking zanamivir and in people taking placebo. We found no good evidence of benefit if zanamivir is started more than two days after symptom onset.

Oral Oseltamivir for Early Treatment of Influenza A and B in Adults (Duration of Symptoms Reduced)

Two RCTs have found that oral oseltamivir reduces the duration of influenza symptoms by about one day compared with placebo. Oral oseltamivir increases the incidence of nausea and vomiting compared with placebo. We found no good evidence of benefit if oseltamivir is started more than one and one half days after symptom onset.

Oral Rimantadine for Early Treatment of Influenza A in Adults (Duration of Symptoms Reduced)

One systematic review has found that oral rimantadine reduces the duration of influenza A symptoms by about one day compared with placebo. We found insufficient evidence about adverse effects in this setting. We found no good evidence of benefit if rimantadine is started more than two days after symptom onset.

UNKNOWN EFFECTIVENESS

All Antivirals (Reduction of Serious Influenza Complications)

We found insufficient evidence about the effects of antiviral agents on reducing serious complications of influenza.

Definition

Influenza is caused by infection with influenza viruses. Uncomplicated influenza is characterized by the abrupt onset of fever, chills, nonproductive cough, myalgias, headache, nasal congestion, sore throat, and fatigue.¹ Influenza usually is diagnosed clinically. Not all people infected with influenza viruses become symptomatic. People infected with other pathogens may have symptoms identical to those of influenza.² The percentage of infections resulting in clinical illness can vary from about 40 to 85 percent, depending on age and preexisting immunity to the virus.³

Influenza can be confirmed by viral culture, immunofluorescence staining, enzyme immunoassay, or rapid diagnostic testing of nasopharyngeal, nasal, or throat swab specimens, or by serologic testing of paired sera. Some rapid tests detect influenza A only, some detect and distinguish between influenza A and B, whereas others detect but do not distinguish between influenza A and B.

Incidence/Prevalence

In temperate areas of the northern hemisphere, influenza activity typically peaks between late December and early March, whereas in temperate areas of the southern hemisphere, influenza activity typically peaks between May and September. In tropical areas, influenza can occur throughout the year.² The annual incidence of influenza varies yearly and depends partly on the underlying level of population immunity to circulating influenza viruses.¹ One localized study in the United States found that serologic conversion with or without symptoms occurred in 10 to 20 percent per year, with the highest infection rates in people younger than 20 years.⁴ Attack rates are higher in institutions and in areas of overcrowding.⁵

Etiology/Risk Factors

Influenza viruses are transmitted primarily from person to person through respiratory droplets disseminated during sneezing, coughing, and talking.^1,6

Prognosis

The incubation period of influenza is one to four days, and infected adults are usually contagious from the day before symptom onset until five days after symptom onset. The signs and symptoms of uncomplicated influenza usually resolve within one week, although cough and fatigue may persist.¹ Complications include otitis media, bacterial sinusitis, secondary bacterial pneumonia and, less commonly, viral pneumonia and respiratory failure. Complications also are caused by exacerbation of underlying disease.^1,2 In the United States each year, more than 110,000 admissions to the hospital and about 20,000 deaths are related to influenza.² The risk of hospitalization is highest in people 65 years or older, in very young children, and in those with chronic medical conditions.^1,7,8 More than 90 percent of influenza-related deaths during recent seasonal epidemics in the United States have been in people 65 years or older.¹ During influenza pandemics, morbidity and mortality may be high in younger age groups.¹ Severe illness is more common with influenza A infections than with influenza B infections.¹ Influenza immunization is covered in Clinical Evidence under community-acquired pneumonia.