Seasonal Influenza

Seasonal Influenza Statistics

The annual number of influenza-associated deaths over the last four decades in the United States has ranged from 3349 to 48,614. For adults, increased rates of morbidity and mortality are associated with older age and concurrent underlying comorbidities.

Seasonal Influenza and the Elderly

Influenza epidemics generally affect elderly persons disproportionately, with the elder population presenting with the highest rates of morbidity and mortality. Another important risk factor is excess hospitalization. Patients with chronic diseases that require hospitalization have greater risk of acquiring influenza. Rates of hospitalization for acute cardiopulmonary events and mortality were higher during the influenza season and the presence of other comorbidities increased the risk of hospitalization and death. Studies assessing the Influenza vaccine demonstrated a decrease in hospitalizations for cardiac disease and cerebrovascular disease among a large cohort of patients 65 years and older compared to members who were not vaccinated. However, influenza vaccine optimization is challenging as the influenza strain rapidly mutates as it moves from continent to continent.

Why Does Seasonal Influenza Have a Greater Impact on the Elderly?

Factors that contribute to more severe influenza infections in elderly patients include decreased lung compliance, decreased respiratory muscle strength, declining cellular immunity, and decreased B cell responses to new antigens.

Seasonal Influenza and the Elderly Death Rates

Although death rates from influenza are usually disproportionately higher among elderly individuals and infants during influenza epidemics, a shift in the age distribution is seen during pandemics. During the 1918 influenza pandemic, high rates of mortality occurred in young adults between the ages of 20 and 40 years, in addition to affecting the older populations. The 2009 pandemic showed a similar trend, with high rates of morbidity and mortality among children and young adults.

Seasonal Influenza in the United States

Between May and September in 2014, the World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System collaborating laboratories in the United States tested 66,006 specimens for influenza; 3,209 (4.9%) were positive for influenza. Of the 3,209 specimens positive for influenza, 1,728 (54%) were positive for influenza A viruses and 1,481 (46%) for influenza B viruses.

In 2014, the percentage of deaths attributed to pneumonia and influenza ranged from 5.2% to 6.0%. Five influenza-associated pediatric deaths occurring during May 18–September 20 were reported; two were associated with an influenza A (H3N2) virus, one was associated with an influenza A virus for which no subtyping was performed, and two were associated with an influenza B virus.

Novel Influenza A Virus Infection

During May 18–September 20, two human infections with H3N2v viruses were reported by Ohio. Both patients recovered, but one of the two patients was hospitalized as a result of H3N2v illness. In both instances, direct contact with swine in the week preceding illness onset was reported. No ongoing community transmission of these viruses has been detected.

Seasonal Influenza Across the Globe

Common seasonal patterns of influenza activity occur in temperate climate Southern Hemisphere countries. In Australia and New Zealand, influenza activity began to increase in late July and remained elevated through mid-September. Influenza A viruses predominated in both countries. In South Africa, influenza activity begins to increase in late May and decreases in early August. In temperate countries of South America, influenza activity begins to increase in June and remains elevated through July and mid-August, and decreases in September. Influenza A viruses, in 2014, were reported more frequently than influenza B viruses, and influenza A (H3N2) viruses were predominant in Chile, Argentina, Uruguay, and Paraguay. In temperate climate countries of Europe and North America, influenza activity is low, and small numbers of pH1N1, influenza A (H3N2), and influenza B viruses were identified.

In countries with tropical influenza seasonality, overall influenza activity remains low. In the Caribbean and Central America, an increase in the number of influenza B viruses was reported in July and August, particularly in Honduras, Jamaica, and Nicaragua, with influenza A viruses co-circulating in Guatemala and Panama. In tropical South America, influenza A viruses were most commonly reported. Influenza A (H3N2) viruses predominated in Brazil and Colombia, whereas influenza B viruses were more frequently reported in Ecuador. In Peru, influenza A (H3N2) and pH1N1 viruses cocirculated, but influenza B viruses also were identified.

In South Asia and Southeast Asia, a decrease in influenza activity is observed during August and September, and influenza A (H3N2) predominated in Cambodia, India, China, and Vietnam, with smaller numbers of influenza B viruses reported. In Thailand, influenza B viruses is frequently reported in July and August, but influenza A (H3N2) and pH1N1 viruses are also identified.

Seasonal Influenza Treatments are Strain Specific

There are two classes of antiviral drugs are available for the treatment and prevention of influenza:

  • The neuraminidase inhibitors, zanamivir and oseltamivir, which are active against both influenza A and B.

Seasonal Influenza Treatments and the Risk of Influenza-Resistant Strains

However, instances of antiviral drug resistance among influenza viruses have been reported, and in 2007, oseltamivir (Tamiflu)-resistant strains were first observed. The vast majority of neuraminidase inhibitor resistance affects oseltamivir. The H274Y mutation, which results in oseltamivir resistance, emerged in 2007 and became widespread among H1N1 viruses during the 2008 to 2009 influenza season. Neuraminidase inhibitors interrupt the release of progeny influenza virus from infected host cells, which in turn, prevents viral shedding. Before 2007, oseltamivir resistance was rare in ambulatory care settings and emerged during treatment in 1 to 5 percent of cases in clinical trials. However, in late 2007, resistant isolates emerged in Europe. During the 2007 to 2008 influenza season, the average prevalence of resistance to oseltamivir in H1N1 isolates from Europe was 20 percent, with the highest rate (67 percent) reported in Norway. None of the patients included in the initial reports of oseltamivir-resistant influenza had been taking oseltamivir, suggesting that resistant virus can be transmitted between individuals. Lower rates of oseltamivir-resistant H1N1 influenza A virus were found in other regions (26 percent in Canada, 11 percent in the United States, 3 percent in Japan) between the last quarter of 2007 and June 2008. However, rates of resistance will continue to increase, demanding newer forms of therapeutic intervention. For instance, between October 2008 and January 2009, 185 of 190 H1N1 influenza A virus isolates (97 percent) tested from 30 states in the United States were resistant to oseltamivir. All isolates were susceptible to zanamivir, amantadine, and rimantadine. High rates of oseltamivir resistance were also observed among H1N1 influenza isolates in many other regions, including South Africa (100 percent), Australia (93 percent), and the Philippines (91 percent) during the 2008 to 2009 influenza season.

Infectious Diseases Tuberculosis

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