1

A day before someone gets symptoms of flu, it can already been detected that the person will get the flu, as the person is already contagious. Would an antiviral like oseltamivir (Tamiflu) then be able to prevent the flu?

2

As soon as a virus is replicating effective antiviral drugs can hamper or even prevent its progression.

This principle is used for Pre-exposure prophylaxis, and more like in this case in question in Post-exposure prophylaxis.

With something like Oseltamivir it should work similarly.

Clinical Question: How effective are oseltamivir and zanamivir at decreasing post-exposure transmission of influenza?
(Tools for Practice, December 5, 2016)
Bottom Line: For institutionalized seniors, six weeks of oseltamivir or 14 days of zanamivir or will prevent one additional influenza case in every 25-27 treated. For every 7-8 households given post-exposure prophylaxis (PEP), one household will avoid anyone developing influenza.

Resistant influenza A viruses in children treated with oseltamivir: descriptive study. (Lancet, 2004)
Oseltamivir-resistant mutants in children being treated for influenza with oseltamivir arise more frequently than previously reported. Furthermore, children can be a source of viral transmission, even after 5 days of treatment with oseltamivir.

The current position of the RKI still recommends the use of this substance 48 hours after the appearance of the first actual symptoms and goes on to recommend its usage if the progression is thought to be severe.

But there is a huge catch in general with this substance specifically:

Cochrane team comments on the ECDC draft advice on oseltamivir: Criticising misinterpretation in the draft “Expert Opinion” (MedCheck (April 2016, Vol. 2, No. 4))

As ECDC advice and expert opinion on neuraminidase inhibitor have many limitations including misunderstanding of the most important findings of our systematic review, meta-analysis and discussions.
We strongly recommend that our Cochrane review be re-read.
Findings from epidemiological studies should be taken into account.
Findings from basic sciences are also important to understand the mechanism of efficacy and harm from neuraminidase inhibitors:
Inhibition of host’s neuraminidase followed by impaired functions of various cell such as immune, metabolic, renal, cardiac and neuronal cells by neuraminidase inhibitors is closely related not only to the symptom relief but also many adverse effects on various organs.
Central nervous system depressing and stimulating actions of oseltamivir but not zanamivir may be closely related to abnormal behaviours and sudden death from respiratory failure after oseltamivir use.
Finally, we find it strange that a public body would dismiss the findings of our Cochrane review and align its conclusions with a pharmaceutically-sponsored meta-analysis for which neither protocol nor assessment of risk of bias seems to exist.

There is an effect observed for this drug but how reliable these findings are remains elusive: "Prophylaxis trials showed that oseltamivir and zanamivir reduced the risk of symptomatic influenza in individuals and households. There was no evidence of an effect on asymptomatic influenza or on non-influenza, influenza-like illness, but trial conduct problems prevent any definitive conclusion."

Some examples for serious problems in the 'research' conducted:

Oseltamivir and zanamivir have limited effect on symptoms and do not reduce hospitalisation or serious complications of influenza (Evidence Based Medicine December 2014, 19, 6)
The authors identified 20 CTRs of oseltamivir with 9623 participants, and 26 CTRs of zanamivir with 14 628 participants; many of these studies were never published. The exhaustive review of regulatory documents uncovered some important and previously unknown threats to validity of these clinical trials. For example, in 11 of 20 oseltamivir trials, the placebo and active drug capsules were not identical, while ascertainment and reporting of complications and hospitalisations was inconsistent or incomplete in many trials, and participants who did not complete the trials did not receive any of the $300 payment for participation, a failure which constitutes an ethical breach.
Regarding symptom duration, they found only a modest reduction in symptom duration (0.6 days for zanamivir and 0.7 days for oseltamivir), less than that described in the published reports. Oseltamivir did not reduce the likelihood of hospitalisation (risk difference (RD)=0.07%, 95% CI −0.78% to 0.44%), an outcome not reported in the zanamivir trials.
No difference was found in the likelihood of serious complications with either drug. While a small reduction in the likelihood of pneumonia was seen in six studies with 2316 patients (RD=1.0%, 95% CI 0.2% to 1.49%), the diagnosis was not radiographically confirmed, was inconsist- ently defined and relied on self-report by the patient or physician, mediated by the trialists. Nausea (RD=3.7%, 95% CI 0.9% to 7.4%) and vomiting (RD=4.6%, 95% CI 2.4% to 7.6%) were more common in adults receiving oseltamivir. This was also seen in children with oseltamivir, but not with zanamivir. Both drugs were found to be effective for prophylaxis of influenza, with a number needed to treat of 33 for oseltamivir and 51 for zanamivir.

The publication bias displayed here alone should be grounds to seriously doubt the findings in relation to this drugs over all.

Neuraminidase inhibitors for preventing and treating influenza in adults and children (Cochrane Database of Systematic Reviews 2014, Issue 4. Art. No.: CD008965. DOI: 10.1002/14651858.CD008965.pub4.) Oseltamivir and zanamivir have small, non-specific effects on reducing the time to alleviation of influenza symptoms in adults, but not in asthmatic children. Using either drug as prophylaxis reduces the risk of developing symptomatic influenza. Treatment trials with oseltamivir or zanamivir do not settle the question of whether the complications of influenza (such as pneumonia) are reduced, because of a lack of diagnostic definitions. The use of oseltamivir increases the risk of adverse effects, such as nausea, vomiting, psychiatric effects and renal events in adults and vomiting in children. The lower bioavailability may explain the lower toxicity of zanamivir compared to oseltamivir. The balance between benefits and harms should be considered when making decisions about use of both NIs for either the prophylaxis or treatment of influenza. The influenza virus-specific mechanism of action proposed by the producers does not fit the clinical evidence.

There are some known problems regarding the general efficacy of this drugs, which are the result of improper conduct in science and marketing on behalf of the manufacturer:

Oseltamivir: over 15 years of data retention and systematic stonewalling (Prescrire International February 2016/Volume 25 n° 168 • Page 55)
In summary. Over a 15-year period, Roche, the company that markets Tamiflu°, has hampered independent analysis of the assessment data on oseltamivir in the treatment of influenza. Worse yet, drug regulators and international organisations have been complicit in this data retention.
These 15 years of stonewalling represent a lost opportunity for patients and the medical community, while providing the company with an unfair advantage after successfully bringing oseltamivir to the market on the basis of unverified data. Yet, despite the widespread belief in the efficacy of oseltamivir, cleverly orchestrated by the company and other organisations, including some regulatory and health authorities, a number of independent teams were not convinced and instead went on a hunt for missing data. What they discovered was that the available trial results were neither complete nor clinically relevant and pro- vided only weak evidence. Furthermore, they revealed that the company had not provided the information theoretically required to obtain marketing authorisation.
From the company’s point of view, this deception was a success. Once health authorities and health professionals had been convinced that oseltamivir was effective on influenza, each new assessment that came to a dissenting or unfavourable conclusion could be countered by publication of a new company-funded analysis. This widely used strategy is called a “publication plan” (17).

Conclusion

Given the above reasoning a working drug, an effective drug will prevent outbreaks of symptoms as described. But Oseltamivir seems to be not very efficacious in this regard. The small effect, if any, this drug may present seems to be in relation to the side effects more in the not worth it drawer than anything else.

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