2018–2019 Influenza Season: What We Know So Far

The influenza season in the WHO European Region is picking up and WHO/Europe is providing regular updates of the situation based on data provided by Member States.



Knowing how influenza is spreading through the Region (often in a west–east pattern) and which strains of the virus predominate helps prepare countries for the peak of the season, when health services may become overburdened by the number of patients.

Influenza type A viruses: most common this season
There are 2 types of influenza viruses that cause illness in humans, type A and type B. Currently, the 2 seasonal influenza A virus subtypes are circulating in the Region in almost equal proportions, with A(H1N1)pdm09 viruses being slightly more prevalent than A(H3N2). Very few influenza B viruses have been detected. The distribution of viruses can change over the course of a winter, and the current distribution may not remain the same.

Numbers of deaths associated with influenza as expected
Data from the 23 Member States and areas reporting to European Monitoring of Excess Mortality for Public Health Action (EuroMOMO) indicate that all-cause mortality is at expected levels for this time of year, but with a few countries starting to observe some excess mortality in elderly populations.

Vaccine effectiveness: too early to tell
Laboratory analyses to determine the similarity of circulating influenza A(H1N1)pdm09 and A(H3N2) viruses to the vaccine strains are ongoing. Very few influenza B viruses have been reported and it is too early to say which lineage (Victoria, included in trivalent vaccines, or Yamagata, included in quadrivalent vaccines) will predominate. It is too early in the season to have data on vaccine effectiveness, which looks at how well the vaccine protects against clinical illness. Countries should continue to encourage vaccination.

Who is at risk
During the winter months, influenza may infect up to 20% of the population, depending on which viruses are circulating. People at increased risk of severe disease once infected include the elderly, pregnant women, young children, immune-compromised people and people with chronic underlying medical conditions. These groups represent a significant proportion of the population in the European Region.

WHO recommends that everyone at risk of developing severe disease because of infection with influenza, as well as health-care workers, be offered seasonal influenza vaccination. As it is not possible to predict if one influenza virus will predominate in a particular season, seasonal influenza vaccines should cover all viruses that are anticipated to circulate, namely influenza A(H3N2), influenza A(H1N1) and influenza B.

Health providers should suspect and treat severe influenza in people at high risk of severe disease, and should consider the use of influenza-specific antiviral medicines even in patients who have been vaccinated.

Flu News Europe: surveillance and updates
Countries conduct surveillance to characterize the circulating influenza viruses, to determine the timing of the influenza season and the potential severity of disease, and to provide data to WHO for regional and global updates. WHO/Europe and the European Centre for Disease Prevention and Control collaborate to collect and analyse influenza surveillance data from Member States in the European Region and present these data each week in the Flu News Europe bulletin.

As the influenza season progresses in 2019, Flu News Europe will continue to report on the situation

Source

 

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Effects of Influenza Vaccination in the United States during the 2017–2018 Influenza Season
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Clinical Infectious Diseases, ciz075, https://doi.org/10.1093/cid/ciz075
Published:

02 February 2019

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Abstract
Background
The severity of the 2017–2018 influenza season in the U.S. was high with influenza A(H3N2) viruses predominating. We report influenza vaccine effectiveness (VE) and estimate the number of vaccine prevented influenza-associated illnesses, medical visits, hospitalizations, and deaths for the 2017–2018 influenza season.

Methods
We used national age-specific estimates of 2017–2018 influenza vaccine coverage and disease burden. We estimated VE, and 95% confidence intervals (CI), against medically-attended RT-PCR confirmed influenza virus infection, in the ambulatory setting, using a test-negative design. We estimated influenza type/subtype-specific burden using multipliers applied to population-based rates of influenza-associated hospitalizations. We used a compartmental model to estimate numbers, with 95% credible intervals (CrI), of influenza-associated outcomes prevented by vaccination.

Results
The VE against outpatient medically-attended, laboratory-confirmed influenza was 38% (95% CI: 31–43%) including 22% (95% CI: 12–31%) against influenza A(H3N2), 62% (95% CI: 50–71%) against influenza A(H1N1)pdm09, and 50% (95% CI: 41–57%) against influenza B. We estimated that influenza vaccination prevented 7.1 million (95% CrI: 5.4 million–9.3 million) illnesses, 3.7 million (95% CrI: 2.8 million–4.9 million) medical visits, 109,000 (95% CrI: 39,000–231,000) hospitalizations, and 8,000 (95% CrI: 1,100–21,000) deaths. Vaccination prevented 10% of expected hospitalizations overall and 41% among young children (6 months–4 years).

Conclusions
Despite 38% VE, influenza vaccination reduced a substantial burden of influenza-associated illness, medical visits, hospitalizations, and deaths in the U.S. during the 2017–2018 season. Our results demonstrate the benefit of current influenza vaccination and the need for improved vaccines.

 

Influenza A (H1N2) Reassortant Infection in Sweden

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Influenza Virus Reassortment:

Reassortment happens when two or more influenza viruses infect a single host and exchange gene segments. Genetic sequencing showed that the A(H1N2) virus in Sweden was a reassortant containing a similar H1 hemagglutinin gene as circulating, seasonal A(H1N1)pdm09 viruses and an N2 neuraminidase gene similar to circulating, seasonal A(H3N2) viruses.

Human infections with reassortant A(H1N2) viruses have occurred rarely in the past, but these were reassortants of the human, seasonal A(H1N1) virus that circulated prior to emergence of the 2009 A(H1N1)pdm09 virus that triggered a pandemic in 2009. This is the second reassortant of seasonal A(H1N1)pdm09 and seasonal A(H3N2) viruses reported. In 2018, a human infection caused by a reassortant A(H1N2) virus was reported in the Netherlands. Laboratory experiments with previous A(H1N1) reassortants in ferret models suggested that these viruses had limited capacity for transmission, however human-to-human spread is still possible.

Risk Assessment:

This A(H1N2) reassortant virus is thought to pose a health risk similar to other seasonal influenza viruses. The virus has not been detected beyond this one person and current seasonal influenza vaccines would likely offer protection against this virus. Additionally, this virus does not have markers associated with resistance to the neuraminidase inhibitor class of antiviral drugs and, thus, should be susceptible to treatment with the currently recommended drugs oseltamivir, zanamivir and peramivir.