Epidemiological Surveillance of Influenza: Strategies for Early Detection and Effective Response
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The influenza (flu) virus circulates around the world at different times of the year. Based on the start of the primary influenza activity period, countries are assigned to two categories:
- Countries where the primary influenza activity starts after October.
- Countries where the primary influenza activity starts after April.
To be most effective in countries with multiple peaks and residual year-round activity, World Health Organization (WHO) recommends that the seasonal influenza vaccine should be given before the start of the primary period of increased influenza activity.
This international agency also encourages health care providers and public health systems to analyze local surveillance information to assess national and regional seasonality patterns in the country, thereby aiding evidence-based decisions on the timing of vaccination campaigns.
According to a WHO update from June 12, 2024:
- In the Northern Hemisphere, elevated influenza activity continued to be reported in countries in Central America and the Caribbean and in South Asia, primarily due to the influenza A(H3N2) virus. In most temperate countries, influenza activity remained low at interepidemic levels.
- In the Southern Hemisphere, countries in South America, South Africa, and Oceania reported elevated activity. In South America, activity was primarily due to A(H3N2) viruses. A(H1N1)pdm09 viruses predominated in South Africa, and A(H1N1)pdm09 and A(H3N2) viruses cocirculated in Oceania.
In temperate climates, seasonal epidemics mostly occur in winter, whereas in tropical climates, they can appear year-round, causing more irregular outbreaks.
Vaccine
Each year, the US Food and Drug Administration (FDA), the US Centers for Disease Control and Prevention (CDC), WHO, and other public health experts collaborate to review influenza surveillance and laboratory data collected around the world in an effort to identify the strains that could cause the most illness in the upcoming season.
Based on that information and the recommendations of the FDA’s Vaccines and Related Biological Products Advisory Committee, every year the FDA selects the different influenza strains that manufacturers should include in their vaccines for the US population for the upcoming influenza season.
The closer the match between the circulating strains that cause illness and the strains in the vaccine, the greater the protection against influenza.
This season, epidemiologists have observed that one of the influenza B strains has disappeared from circulation. For this reason, the 2024-2025 influenza vaccines will contain only three types of influenza instead of four as in previous years.
The updated trivalent version will include protection against two common influenza A viruses and one influenza B virus.
Due to low vaccination rates, and because the flu vaccine does not provide 100% protection, vaccinated people can still get the flu. The more people who are vaccinated, the greater the community protection. That’s why public health agencies suggest that providers should lead the way in recommending the vaccine.
Vulnerable populations
Although most people survive the flu with no aftereffects, there are medically vulnerable groups of individuals who can develop serious complications from the seasonal flu.
They are:
- Pregnant women
- Children under 5 years, and especially those under 2
- Older adults
- Patients with chronic diseases (such as heart, lung, kidney, metabolic, neurodevelopmental, liver, or hematological diseases) or suppressed immune systems (due to HIV/AIDS, chemotherapy, corticosteroids, or cancer).
As they are exposed to patients, health workers and caregivers run a higher risk of getting and spreading flu viruses, especially to vulnerable individuals. Vaccination helps protect health workers and the people around them.
Viruses and bacteria sometimes adopt a “kick ’em while they’re down” approach, taking advantage of the weakened state of a person’s body to cause further infections.
Pathogens that cause a second infection are often called “opportunistic.” One example is that when people have the flu, they are more susceptible to pneumococcal infections.
The CDC estimates that 90% of flu-related deaths occur in adults 65 years and older.Flu vaccination is recommended starting at 6 months, and every provider should discuss the best dose and timing with their patients for optimal protection.
This story was produced using content from original studies or reports, and other medical research and health and public health sources, highlighted in related links throughout the article.
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