The 2017-18 influenza season in the United States began a little more than a month ago, and states have already begun to report their first flu-related deaths, including CaliforniaNorth Carolina, and Nebraska. While the majority of deaths thus far were adults over the age of 65, there has also been one reported pediatric death so far this season.

These deaths are a stark reminder of the dangers of influenza—and the importance of annual vaccination—but the number of deaths attributed to influenza and pneumonia (a common complication of influenza infection) are still below the epidemic threshold at Week 41, as defined by the National Center for Health Statistics Mortality Surveillance System. However, the current low incidence does not necessarily indicate how severe this season will be. For example, during the 2014-15 influenza season, deaths from pneumonia and influenza were below the epidemic threshold at Week 41 but surpassed the threshold just 10 weeks later—this was classified by the CDC as a “moderately severe” season. As a matter of fact, infectious disease experts are already voicing concern that this flu season may be more severe than in previous years.

In today’s blog post, we will look at three reasons why the flu season may be particularly severe this year in the United States:

1) The Southern Hemisphere experienced a particularly bad flu season in 2017, suggesting that we may see a more severe flu strain in circulation in the United States and elsewhere in the Northern Hemisphere
2) The flu vaccine may not provide strong protection against circulating flu strains
3) Vaccination coverage in the United States in recent years has been lower than official targets, suggesting that we could see similarly low coverage this season

A More Severe Flu Strain

It is impossible to predict exactly how the flu season will unfold this year in the United States; however, the Southern Hemisphere influenza season can provide clues as to the severity and strains that we will likely experience. Australia’s 2017 flu season was particularly concerning, as the country saw 217,559 laboratory confirmed notifications of influenza as of October 24—more than twice the number of cases that were reported in calendar year 2016. More than 50 children were hospitalized, and there were more than 500 influenza-related deaths, most of which occurred in adults over the age of 65.

The dominant circulating influenza virus in Australia this season was H3N2, a subtype of influenza A—there are 4 total types of influenza viruses: A, B, C, and D. Influenza seasons where influenza A(H3N2) predominated were found by one study to have the highest rates of influenza-related hospitalizations compared to other influenza viruses. Another study found that A(H3N2) infections resulted in more clinically severe disease than other influenza viruses, including higher fevers and more severe laboratory abnormalities.

Influenza Vaccine Efficacy

A second cause for concern is that influenza vaccines are frequently less efficacious against influenza A(H3N2) viruses, which could be problematic if this is the dominant strain this year in the United States. Efficacy for flu vaccines drops when the virus undergoes antigenic drift, which are small genetic changes in the virus over time. As these genetic changes accumulate, the vaccine and the human immune system are less able to recognize the virus, increasing the probability of infection and disease. According to the CDC, all influenza viruses under antigenic drift, but “the antigenic changes that occur in influenza A(H3N2) viruses have more frequently resulted in differences between the virus components of the flu vaccine and circulating influenza viruses.” So from the time that the decision is made to include this subtype in the next season’s vaccine—in the Northern Hemisphere, typically February each year—and the time the vaccine is delivered, the circulating strain may “have changed in ways that could impact how well the flu vaccine works.” Additionally, most seasonal influenza vaccines are grown in chicken eggs. Influenza A(H3N2) viruses are more likely to undergo antigenic changes during this particular production process, termed “egg-adapted changes.” One recent study demonstrated that these egg-adapted changes elicited “antibodies that poorly neutralize H3N2 viruses that circulated during the 2016–2017 influenza season,” with an overall vaccine effectiveness of 43%. This year’s H3N2 vaccine component is identical to the one used last year, which could result in similar levels of effectiveness in the event that influenza A(H3N2) is in high circulation this year.

Other vaccine production technologies are available that could potentially circumvent some of the challenges faced when making the H3N2 component of the influenza vaccine. Cell-based vaccines use animal cells for growing influenza virus rather than fertilized chicken eggs and, according to the CDC, these processes “reduce egg-adapted changes and may result in vaccines containing virus that is more ‘like’ wild-type circulating viruses.” The cell-based vaccine Flucelvax was approved by the FDA in 2012, but it still requires “egg-adapted influenza viral seeds,” which are then grown in mammalian cells. This year, for the first time, a “cell-grown H3N2 vaccine reference virus was used to produce the H3N2 component” of Flucelvax, which may increase its effectiveness for those who receive this specific vaccine. Recombinant vaccines are another option, which completely eliminate the need for chicken eggs. One study found that RIV4 (Flublok), a quadrivalent recombinant influenza vaccine, demonstrated “improved protection against laboratory-confirmed influenza-like illness” compared to egg-based vaccines.

Influenza Vaccine Coverage

Low influenza vaccination coverage in previous years is another reason the United States may be in for a particularly severe flu season. Influenza vaccination is the most effective way of preventing influenza, and it is particularly important for those who are more likely to experience influenza-associated complications such as pneumonia, including the elderly, children, pregnant women, and those with chronic health conditions. Influenza vaccine coverage for the 2016-17 season was well below the targets set for Healthy People 2020, and similar coverage this year could put the United States at increased risk for a severe flu season. While this year’s vaccine may not be a perfect match, vaccination remains an important public health intervention that reduces community transmission and disease severity. As such, public health practitioners must continue to advocate for influenza vaccination, especially in those populations that are at increased risk for complications.


Outbreak Observatory aims to collect information on challenges and solutions associated with outbreak response and share it broadly in near-real time to allow others to learn from these experiences in order to improve global outbreak response capabilities.

Photo courtesy of CDC/Douglas Jordan, M.A.