What is the efficacy of Oseltamivir (Tamiflu)?

From the Guidelines

Tamiflu (oseltamivir) is recommended for the treatment of influenza, particularly for high-risk patients, as it can reduce the duration of flu symptoms and the risk of complications, as shown in a study published in 2019 1. When started within 48 hours of symptom onset, Tamiflu can reduce the duration of flu symptoms by about 1-1.5 days. The standard adult dosage is 75 mg twice daily for 5 days for treatment, or once daily for 10 days for prevention after exposure. For children, dosing is weight-based, with the following recommendations:

• 15 kg (33 lb) or less: 30 mg twice daily
• 15-23 kg (33-51 lb): 45 mg twice daily
• 23-40 kg (51-88 lb): 60 mg twice daily
• 40 kg (88 lb) or more: 75 mg twice daily Tamiflu works by inhibiting viral neuraminidase, preventing new virus particles from being released from infected cells. It's most effective when started early, ideally within 24 hours of symptoms. Side effects may include nausea, vomiting, and headache. Tamiflu is not a substitute for flu vaccination and works only against influenza viruses, not other viral respiratory infections. The medication's benefits should be weighed against its cost and potential side effects for otherwise healthy individuals with mild illness. Some studies, such as one published in 2008 1, suggest that oseltamivir can reduce severe clinical outcomes in patients hospitalized with influenza, but the evidence is not as strong as that for the 2019 study 1. Overall, the use of Tamiflu should be guided by clinical judgment, taking into account the patient's underlying conditions, disease severity, time since symptom onset, and local influenza activity. In addition, antiviral treatment should be started as soon as possible after illness onset and should not be delayed while waiting for a definitive influenza test result, as early therapy provides the best outcomes.

From the FDA Drug Label

The efficacy of oseltamivir phosphate in preventing naturally occurring influenza illness has been demonstrated in three seasonal prophylaxis (community outbreak) clinical trials and one post-exposure prophylaxis trial in household contacts In a pooled analysis of two seasonal prophylaxis trials in healthy unvaccinated adults (aged 18 to 65 years), oseltamivir phosphate 75 mg once daily taken for 42 days during a community outbreak reduced the incidence of laboratory-confirmed clinical influenza from 5% (25/519) for the placebo group to 1% (6/520) for the oseltamivir phosphate group The incidence of laboratory-confirmed clinical influenza was 4% (12/272) in the placebo-treated subjects compared to less than 1% (1/276) in the oseltamivir phosphate -treated subjects The incidence of laboratory-confirmed clinical influenza was 12% (24/200) in the placebo-treated subjects compared to 1% (2/205) in the oseltamivir phosphate -treated subjects Among household contacts 1 year to 12 years of age not already shedding virus at baseline, the incidence of laboratory-confirmed clinical influenza was lower in the group who received oseltamivir phosphate prophylaxis [3% (3/95)] compared to the group who did not receive oseltamivir phosphate prophylaxis [17% (18/106)] The incidence of confirmed clinical influenza was 3% (7/238) in the placebo group compared with 2% (5/237) in the oseltamivir phosphate group; this difference was not statistically significant. The efficacy of Tamiflu (oseltamivir phosphate) in preventing influenza illness is supported by clinical trials, with a significant reduction in the incidence of laboratory-confirmed clinical influenza in various populations, including:

• Healthy adults: 75 mg once daily reduced the incidence from 5% to 1% 2
• Elderly residents: 75 mg once daily reduced the incidence from 4% to less than 1% 2
• Household contacts: 75 mg once daily reduced the incidence from 12% to 1% 2
• Pediatric subjects: oseltamivir phosphate prophylaxis reduced the incidence from 17% to 3% 2 However, in immunocompromised subjects, the difference in incidence of confirmed clinical influenza between the oseltamivir phosphate group and the placebo group was not statistically significant 2

From the Research

Tamiflu Efficacy

• Tamiflu (oseltamivir) is a prodrug of oseltamivir carboxylate, a potent and selective inhibitor of the neuraminidase glycoprotein essential for replication of influenza A and B viruses 3.
• Studies have shown that oseltamivir reduces the duration of the disease by up to 1.5 days and the severity of illness by up to 38% compared with placebo when initiated within 36 hours of symptom onset 3.
• Oseltamivir has also demonstrated efficacy when used for seasonal or household prophylaxis, preventing the development of naturally acquired influenza by >70% compared with placebo in unvaccinated otherwise healthy adults 3.
• The drug is well tolerated, with nausea and vomiting being the most commonly reported events, and is considered a useful therapeutic alternative to other antiviral medications 3.

Special Populations

• Oseltamivir has been shown to be effective in the treatment of children with influenza, and data suggest that it may also be of use in high-risk populations such as the elderly or those with chronic cardiac or respiratory disease 3, 4.
• However, there is limited information available on the efficacy and safety of oseltamivir in special populations such as critically ill patients and patients with a history of chronic respiratory disease 5.

Resistance and Cross-Resistance

• The emergence of strains with reduced or no susceptibility to oseltamivir and other neuraminidase inhibitors is a concern, and constant monitoring of viral variants is necessary 5, 6.
• Clinical cases of influenza viruses harboring single or multiple NA substitutions or deletions conferring a cross-resistance phenotype to oseltamivir and zanamivir have been reported, mostly in immunocompromised individuals 6.

Comparative Effectiveness

• A study comparing the clinical effectiveness of four neuraminidase inhibitors (oseltamivir, zanamivir, laninamivir, and peramivir) for children with influenza A and B in Japan found that the duration of fever after administration of the first dose of the NAI was shorter in older patients and in patients with influenza A infection than in patients with influenza B infection 7.