"Prediction is very difficult, especially if it's about the future."
Nils Bohr, Nobel laureate in Physics
"Anyone who tries to predict the flu season based on early information doesn’t understand influenza very well. The case numbers are up early, but it's such a difficult disease to predict—I don't think we can say anything about how severe it may be."
Michael Osterholm, Director, Center for Infectious Diseases Research and Policy
You may recall back at the beginning of the 2019-2020 flu season a point that was made in the October 5 Germ Gems post (What’s New with the Flu?) that one of the reasons the annual influenza vaccine isn’t great is because of the difficulty of predicting what flu viruses to include in the vaccine. Well, here we are in the middle of the flu season and this predictably unpredictable situation has come true with an unexpectedly high rate of infections caused by influenza B virus.
In most flu seasons, influenza A viruses predominate (75% of cases vs 25% due to influenza B viruses). According to the Center for Disease Control and Prevention’s (CDC) January 11, 2020 weekly influenza surveillance report, the influenza B/Victoria strain is currently the most common flu strain in the U.S. An early edition of the CDC's Morbidity and Mortality Weekly Report noted that the last time influenza B viruses were the predominant U.S. flu virus was the 1992-1993 flu season. They added that B/Victoria viruses account for 60% of circulating U.S. viruses this year, whereas in the last 3 years, this strain accounted for less than 10% of circulating viruses.
How was the composition of the current flu vaccine determined? You may also recall from the October 5 Germ Gems post that the decision-making process regarding which influenza viruses to include in the annual seasonal flu vaccine is complex. In February of each year the World Health Organization convenes a group of consultants who, based on surveillance data, recommend what viruses to consider for the upcoming flu season which typically begins in October in the Northern Hemisphere.
For example, the 2019-2020 trivalent (three-component) influenza vaccine contains A/Brisbane/02/2018 (H1N1)pdm09-like virus, A/Kansas/14/2017 (H3N2)-like virus, and B/Colorado/06/2017-like (Victoria lineage) virus. The quadrivalent (four-component) influenza vaccine contains an additional influenza B virus. In good years, that is, those years when the predictions turn out to be right, at best a 60% protection can be expected. When the predictions are off, often only 20% protection is achieved.
What are the differences between influenza A and B? Influenza A and influenza B viruses are genera of the same Orthomyxoviridae family of RNA viruses. Both are highly contagious and are spread in droplets by coughing or sneezing. In addition to infecting humans, type A influenza viruses are found in many other animals including ducks, chickens, pigs, whales, horses, seals, and cats. Three devastating worldwide influenza A pandemics got their start as avian flu before jumping to humans. Influenza B viruses, on the other hand, circulate widely only among humans. Type B viruses have never been a source of a flu pandemic. Also, type B viruses mutate more slowly than do type A flu viruses.
Each year, 5% to 20% of all Americans are sickened by the flu and more than 200,000 are hospitalized because of flu-related complications. Since 2010, influenza has killed between 12,000 – 79,000 Americans annually. While type A viruses may cause more severe disease and kill more people, as the current flu season is demonstrating, type B flu can also be extremely harmful. Not only are there more deaths this flu season caused by influenza B, but the virus has a predilection for children.
By the week ending January 11, 2020 a total of 39 influenza-associated pediatric deaths were reported by the CDC. Twenty-eight of these deaths were associated with influenza B viruses. Five of these had the lineage determined and all were B/Victoria viruses. In most years when influenza A predominates, the elderly or those with underlying medical conditions are at greatest risk of dying. But tragically in this flu season, in which influenza B is predominant, many deaths have occurred in previously healthy children or adolescents.
As a rule, influenza viruses cause inflammation in the upper respiratory tract where they provoke the main symptoms of the flu—sore throat, runny or stuffy nose, and dry cough. Other symptoms, such as, fever, myalgias (muscle pains), fatigue, and anorexia (loss of appetite) are mediated by proteins (cytokines) that are released from cells of the immune system as they battle the flu virus. If the infection reaches the lower respiratory tract, pneumonia sets in, and the immune response can become overwhelming (dubbed a cytokine storm).
The treatment of influenza pneumonia is especially challenging. Antiviral medications, such as, oseltamivir (Tamiflu), zanamivir (Relenza), or baloxavir marboxil (Xofluza), are routinely given. When a bacterial superinfection occurs, an antibiotic is administered. But in fatal cases, on-going damage to the lungs or the sepsis syndrome—multiple organ system failure due to a dysregulated immune system— becomes uncontrollable. (Sepsis was the topic of the November 3 Germ Gems post.)
So what should you do now? Even though this year’s influenza vaccine is far from perfect, if you haven’t been vaccinated, you should do so as soon as possible. (The CDC recommends the flu vaccine for everyone 6 months of age or older, unless there is a contraindication to the vaccine.) It’s clearly better than nothing, and the side effects are negligible.
Even if you’ve been vaccinated, if you develop symptoms of the flu (fever and a dry cough occurring at the same time are a tip-off during the flu season), contact your doctor or other health care professional as soon as possible. To be effective, antiviral medicines (oseltamavir for five days or a single dose of baloxavir, both of which are active against influenza A and B viruses) need to be taken within 48 hours of the onset of flu symptoms.
At the end of the 2019-2020 influenza season will influenza A viruses overtake influenza B viruses? It’s anybody’s guess, as are what surprises are in store for the future. But let’s all hope, it’s not another major flu pandemic.
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