Key to Correct Management of Novel Influenza Epidemics

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Influenza is an infection caused by influenza viruses that regularly cause outbreaks in a variety of species, including birds and mammals, including humans. Despite being a known virus, its behavior at the epidemiological level does not prevent complications and health emergencies worldwide. Although in humans it generally represents a mild and reversible process (characterized by fever, respiratory symptoms, and general malaise), it may pose a danger to particularly vulnerable populations. Risk factors that make you more susceptible to influenza include advanced age, pregnancy, chronic underlying medical conditions, neoplastic processes, heart disease, immunosuppressive treatments, and other serious medical conditions.

In these people, the virus may destabilize its basic processes or have particularly lethal behavior, in either case it can have serious and irreversible consequences. Less commonly, perfectly healthy people with no known risk factors may find that their health is severely compromised and require hospitalization. Additionally, these hospitalizations can occur over a short period of time and cause health systems to collapse. For all these reasons, influenza and its consequences cannot be underestimated in terms of both morbidity and mortality. In addition, co-circulation of influenza with the coronavirus SARS-CoV-2, RSV (respiratory syncytial virus), and other respiratory viruses may worsen the baseline status of susceptible patients and create complex epidemic situations with a huge impact on public health.

Probabilistic models of influenza virus transmission are complex. In immunocompetent adults, the potential for transmission is greatest the day before symptoms appear and remains extremely high until fever develops. From the first peak of fever (usually the first 48 or 72 hours after the onset of symptoms), the likelihood of transmission decreases dramatically due to its lower replication capacity. Starting on the fifth day, the probability of transmission is minimal (1) and remains asymptotic between days 7 and 12 (2). In fact, the virus is more transmissible in its early stages, especially in the asymptomatic stage, giving it a very high transmissibility, which is also a characteristic of its biological expansion strategy. Although specific antiviral treatments exist, their effectiveness may be compromised because they must be given when symptoms are present to be effective. Subsequently, treatment is supportive only.

The main mechanism of spread of influenza is through droplets (particles larger than five microns) expelled when talking, sneezing, and coughing. Therefore, establishing a safe distance is crucial to avoid contagion. Another typical basic element of prevention is the observance of standard precautions or basic hygiene rules, the most important of which is hand hygiene with a water and soap solution or an alcohol-based preparation. Another measure is the correct use of masks in particularly vulnerable settings when safe distance cannot be maintained.

To sum up, preventing influenza is an important pillar in controlling this epidemic. The basic prevention strategy is to stop the spread of the virus. For this reason, and given the transmission cycle of the virus, it is important to isolate cases as early as possible during the early stages when the likelihood of transmission is highest. A very common mistake is to think that the virus spreads equally at all stages. Strictness involves isolating cases during the period when transmission is most likely, limiting the duration of isolation to periods when transmission is most likely, thereby making better use of health resources and hospital beds, and shortening flu sick days.

The top of the prevention pyramid is vaccination. On the one hand, vaccination strategies specifically target vulnerable groups because they have one or more risk factors: age, pregnancy, puerperium, chronic pathology, immunodeficiency, neoplastic processes and other serious pathologies. On the other hand, the strategy targets those who can contribute to reducing the spread and spread of the virus. In this sense, starting from this year, in accordance with the recommendations of the World Health Organization at the international level, the population of children aged 6 to 59 months will be targeted. In addition to children being more vulnerable, the purpose of vaccinating children is to stop the spread of the virus in the community and avoid secondary cases among adults who fall into the previously identified risk groups. The third category consists of workers in essential services whose sick leave could lead to critical situations with significant social impact.

The vaccine components during the 2023-24 season in the Northern Hemisphere were selected based on epidemiological studies of the most common virus types in accordance with World Health Organization (WHO) recommendations. Southern Hemisphere dates. Current influenza vaccine formulations contain viruses similar to: A/Victoria/4897/2022 (H1N1)pdm09 (new pandemic variant); A/Darwin/9/2021 (H3N2); B/Austria/1359417/2021 (B /Victorian origin); B/Phuket/3073/2013 (B/Yamagata origin) (3). The nomenclature expressed previously demonstrates the large variability in the types of influenza viruses that can be transmitted a priori and the difficulty in matching the types of viruses that ultimately circulate with predictions.

However, prevention is not just the goal of reducing influenza-related morbidity and mortality. Reducing transmissibility means reducing the chance of genetic changes to the virus that could confer greater virulence and thus increase its morbidity and mortality. Additionally, stopping the spread of the virus reduces the likelihood of it jumping from animal species to humans, preventing the emergence of new, more aggressive variants due to species jumping. This limits the emergence of an outbreak that could lead to a pandemic.

For all these reasons, vaccination, combined with the preventive measures mentioned above, constitutes an effective measure with a significant impact on public health. The benefits of vaccination are not limited to the vaccinated person but extend to their entire environment, helping to reduce the occurrence of secondary cases and control influenza epidemics globally.

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(1) A. Cori a, AJ Vallerona, F. Carrat, et al. Use viral excretion data to estimate influenza incubation and infectious period duration. Epidemics, Volume 4, Issue 3, August 2012, Pages 132-138.

(2) Fabrice Carrat, Elisabeta Vergu, Neil M. Ferguson, et al. American Journal of Epidemiology, 2008, Volume 167, Issue 7.

(3) Directorate of Public Health and Patient Safety Program Coordination Health Assistance, Osakidetza: 2023 Influenza Vaccination Campaign.