Drug update: Influenza.

Antiviral treatment for influenza generally works as a complement to immunization, not as a substitute, for most patients.

People at high risk of serious complications from influenza who have not been vaccinated have the highest priority for prophylactic treatment, and such high-risk patients should also be treated if they become infected. Early diagnosis and prescribing are necessary when treating an infection because treatment is effective only when it is started within 2 days of an infection.

All four drug options appear to be effective for preventing disease in about 80% of patients, but drug-to-drug comparisons are lacking. Treatment may be more effective for alleviating symptoms in high-risk patients and severely ill patients with high fever. No drug has been shown to prevent serious influenza-related complications such as bacterial or viral pneumonia or exacerbation of chronic diseases.

Despite the higher cost of zanamivir and oseltamivir, either of these drugs is the treatment of choice for some experts because they cover both influenza strains, produce fewer side effects, and are less likely to lead to the emergence of drug-resistant viruses. Amantadine, the only generic option, or rimantadine may be the first choice for treatment when there are outbreaks of influenza A and when cost is particularly important. Distinguishing influenza A infection from influenza B infection requires attention to local epidemiologic trends, public health department findings, and laboratory results.

Oseltamivir, amantadine, and rimantadine are approved for influenza prophylaxis. Although they appear to prevent infection about half of the time, they also appear to prevent illness in approximately 70%-90% of people. Rimantadine and amantadine are preferred for prophylaxis against influenza A because of cost considerations; only oseltamivir protects against influenza B. The duration of prophylaxis differs from the duration of treatment for an acute infection.

When choosing a drug, dose, and duration of therapy consider the patient's age, weight, renal and hepatic function, the presence of other medical conditions, potential interactions with other medications, and whether the goal is prophylaxis or treatment of acute illness. All four drugs are in pregnancy category C and are not recommended for use during breast-feeding. No clinical studies have been conducted in pregnant women. Use the drugs during pregnancy only if the potential benefit justifies the potential risk. High doses of amantadine and rimantadine are teratogenic and embryotoxic in animals.

 
Drug          Dosage         Cost/Day (*) 
 
oseltamivir   75 mg b.i.d.   $11.90 
 (Tamiflu)                   (capsule) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
zanamivir     10 mg b.i.d.   $9.60 
 (Relenza) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
rimantadine   100 mg b.i.d.  $4.04 
 (Flumadine)                 (tablet) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
amantadine    100 mg b.i.d.  $0.82 (**) 
                             (capsule) 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Drug          Comment (+) 
 
oseltamivir   Capsules and oral suspension available. 
 (Tamiflu)    Listed dosage is for treatment of influenza; 
              prophylactic dosage is 75 
              mg/day. Reduce dosage to 75 mg/day 
              for treatment or 75 mg every 2 days for 
              prophylaxis in patients with 
              creatinine clearance of less than 
              30 mL/min. Adverse effects, occurring 
              in about 10% of patients, include 
              headache, nausea, and vomiting and 
              may be minimized if the drug is 
              given with food. May interact with other 
              drugs excreted by the anionic pathway 
              such as probenecid. Like zanamivir, a 
              neuraminidase inhibitor that's 
              effective against all strains of influenza 
              A and B. Reduces duration of symptoms 
              by about 1-1.5 days if begun 
              within 30-36 hours of infection. Despite 
              higher cost, zanamivir and oseltamivir 
              usually are the treatments of 
              choice because they cover both influenza 
              strains, produce fewer side effects, and 
              are less likely to lead to the 
              emergence of drug-resistant viruses. 
              Resistant virus has been seen, but the 
              rate is unknown. The biggest 
              difference between zanamivir and 
              oseltamivir is the route of delivery. Limit 
              treatment to 5 days. Some physicians 
              may treat severely ill patients for longer 
              than 5 days, despite a lack of data 
              showing that longer treatment boosts 
              efficacy. Treatment beyond 10-14 
              days probably gives no further benefit. 
              Prophylaxis is for the duration of 
              exposure, usually 6-8 weeks, or for 
              10-14 days after a known exposure. 
 
zanamivir     Used ...