Although there are no randomized controlled studies, several observational studies support  the benefit of NAIs even when initiated after 48 h of onset of symptoms. 

Although the sooner NAIs are initiated the more likely the odds of a favorable impact on the course of influenza, the FDA approval of these drugs was based on analysis of data in relatively healthy ambulatory patients not those who are often sicker and require hospitalization. 

A retrospective study reported improvement in survival even when treatment was delayed for 4-5 days after symptom onset (1). Other studies have reported more rapid viral clearance and clinical benefit in severe infections even when antivirals were initiated after 48 h (2).  

Collectively , these data suggest that in the presence of ongoing symptoms and likely active viral replication, NAI treatment should be seriously considered in hospitalized patients who are likely to have more severe disease.

In fact, CDC recommends  “initiation of antiviral treatment as early as possible” in hospitalized patients with influenza, and asserts that “antiviral treatment might be effective in reducing morbidity and mortality in hospitalized patients even if treatment is not started until >48 hours after onset of illness” (3).  

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References

1. Louie JK, Yang S, Acosta M, et al. Treatment with neuraminidase inhibitors for critically ill patients with influenza A (H1N1) pdm09. Clin Infect Dis 2012;44:1198-1204. https://www.ncbi.nlm.nih.gov/pubmed/22843781
2. Lee N, Ison MG. “Late” treatment with neuraminidase inhibitors for severely ill patients with influenza: better late than never? Clin Infect Dis 2012;55:1205-8. https://www.ncbi.nlm.nih.gov/pubmed/22843780
3. CDC. Antiviral agents for the treatment and chemoprophylaxis of influenza: recommendations of the Advisory Committee on Immunization Practices. MMWR 2011;60 (RR01):1-24.