Influenza update 2018–2019: 100 years after the great pandemic

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Four influenza pandemics, starting with the historic 1918 pandemic, have killed thousands of people around the world. Vaccination, still the most important means of preventing influenza, is currently recommended yearly for all people age 6 months and older, with a goal of vaccinating 80% of all Americans and 90% of at-risk populations. Neuraminidase inhibitors are underused, and a new drug with a different mechanism of action, baloxavir marboxil, is expected to be approved soon in the United States.


  • Influenza A(H7N9) is a prime candidate to cause the next influenza pandemic.
  • Influenza vaccine prevents 300 to 4,000 deaths in the United States every year.
  • The 2018–2019 quadrivalent influenza vaccine contains updated A(H3N2) and B/Victoria lineage components different from those in the 2017–2018 Northern Hemisphere vaccine.
  • The live-attenuated influenza vaccine, which was not recommended during the 2016–2017 and 2017–2018 influenza seasons, is recommended for the 2018–2019 influenza season.
  • Influenza vaccine is recommended any time during pregnancy and is associated with lower infant mortality rates.
  • Overall influenza vaccination rates remain below the 80% target for all Americans and 90% for at-risk populations.



Table 1. Key influenza-related events since the 1918 influenza pandemic
This year marks the centennial of the 1918 influenza pandemic, which killed 50 million people worldwide. Three more influenza pandemics and annual epidemics have occurred since then, with other significant interim events (Table 1).1 The 2017–2018 epidemic was particularly severe and long.2

This centennial year update focuses primarily on immunization, but also reviews epidemiology, transmission, and treatment.


2017–2018 was a bad season

The 2017–2018 influenza epidemic was memorable, dominated by influenza A(H3N2) viruses with morbidity and mortality rates approaching pandemic numbers. It lasted 19 weeks, killed more people than any other epidemic since 2010, particularly children, and was associated with 30,453 hospitalizations—almost twice the previous season high in some parts of the United States.2

Regrettably, 171 unvaccinated children died during 2017–2018, accounting for almost 80% of deaths.2 The mean age of the children who died was 7.1 years; 51% had at least 1 underlying medical condition placing them at risk for influenza-related complications, and 57% died after hospitalization.2

Recent estimates of the incidence of symptomatic influenza among all ages ranged from 3% to 11%, which is slightly lower than historical estimates. The rates were higher for children under age 18 than for adults.3 Interestingly, influenza A(H3N2) accounted for 50% of cases of non-mumps viral parotitis during the 2014–2015 influenza season in the United States.4

Influenza C exists but is rare

Influenza A and B account for almost all influenza-related outpatient visits and hospitalizations. Surveillance data from May 2013 through December 2016 showed that influenza C accounts for 0.5% of influenza-related outpatient visits and hospitalizations, particularly affecting children ages 6 to 24 months. Medical comorbidities and copathogens were seen in all patients requiring intensive care and in most hospitalizations.5 Diagnostic tests for influenza C are not widely available.

Dogs and cats: Factories for new flu strains?

While pigs and birds are the major reservoirs of influenza viral genetic diversity from which infection is transmitted to humans, dogs and cats have recently emerged as possible sources of novel reassortant influenza A.6 With their frequent close contact with humans, our pets may prove to pose a significant threat.

Obesity a risk factor for influenza

Obesity emerged as a risk factor for severe influenza in the 2009 pandemic. Recent data also showed that obesity increases the duration of influenza A virus shedding, thus increasing duration of contagiousness.7

Influenza a cardiovascular risk factor

Previous data showed that influenza was a risk factor for cardiovascular events. Two recent epi­demiologic studies from the United Kingdom showed that laboratory-confirmed influenza was associated with higher rates of myocardial infarction and stroke for up to 4 weeks.8,9

Which strain is the biggest threat?

Predicting which emerging influenza serotype may cause the next pandemic is difficult, but influenza A(H7N9), which had not infected humans until 2013 but has since infected about 1,600 people in China and killed 37% of them, appears to have the greatest potential.10

National influenza surveillance programs and influenza-related social media applications have been developed and may get a boost from technology. A smartphone equipped with a temperature sensor can instantly detect one’s temperature with great precision. A 2018 study suggested that a smartphone-driven thermometry application correlated well with national influenza-like illness activity and improved its forecast in real time and up to 3 weeks in advance.11


Humidity may not block transmission

Animal studies have suggested that humidity in the air interferes with transmission of airborne influenza virus, partially from biologic inactivation. But when a recent study used humidity-controlled chambers to investigate the stability of the 2009 influenza A(H1N1) virus in suspended aerosols and stationary droplets, the virus remained infectious in aerosols across a wide range of relative humidities, challenging the common belief that humidity destabilizes respiratory viruses in aerosols.12

One sick passenger may not infect the whole plane

Transmission of respiratory viruses on airplane flights has long been considered a potential avenue for spreading influenza. However, a recent study that monitored movements of individuals on 10 transcontinental US flights and simulated inflight transmission based on these data showed a low probability of direct transmission, except for passengers seated in close proximity to an infectious passenger.13


The 2018–2019 quadrivalent vaccine for the Northern Hemisphere14 contains the following strains:

  • A/Michigan/45/2015 A(H1N1)pdm09-like virus
  • A/Singapore/INFIMH-16-0019/2016 (H3N2)-like virus
  • B/Colorado/06/2017-like virus (Victoria lineage)
  • B/Phuket/3073/2013-like virus (Yamagata lineage).

The A(H3N2) (Singapore) and B/Victoria lineage components are new this year. The A(H3N2) strain was the main cause of the 2018 influenza epidemic in the Southern Hemisphere.

The quadrivalent live-attenuated vaccine, which was not recommended during the 2016–2017 and 2017–2018 influenza seasons, has made a comeback and is recommended for the 2018–2019 season in people for whom it is appropriate based on age and comorbidities.15 Although it was effective against influenza B and A(H3N2) viruses, it was less effective against the influenza A(H1N1)pdm09-like viruses during the 2013–2014 and 2015–2016 seasons.

A/Slovenia/2903/2015, the new A(H1N1)pdm09-like virus included in the 2018–2019 quadrivalent live-attenuated vaccine, is significantly more immunogenic than its predecessor, A/Bolivia/559/2013, but its clinical effectiveness remains to be seen.


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