What is Influenza B? Understanding the Current Flu Season’s Rising Strain

As flu season progresses, public health officials are monitoring a notable shift in the viral landscape. While Influenza A has historically dominated seasonal outbreaks, recent surveillance data from the Centers for Disease Control and Prevention (CDC) and other global health agencies indicates a significant and sustained increase in the proportion of illnesses caused by Influenza B. This guide provides a clear, evidence-based exploration of Influenza B, explaining its biology, how it differs from its more famous counterpart, current epidemiological trends, and what this means for your health and prevention strategies.

Key Points: Influenza B at a Glance

• Rising Prevalence: For several consecutive weeks, Influenza B has accounted for an increasing percentage of flu cases nationally, recently comprising approximately one-third of all reported influenza infections in the United States.
• Two Lineages: Influenza B viruses are categorized into two primary genetic lineages: B/Victoria and B/Yamagata. The B/Yamagata lineage is currently considered extinct due to successful global vaccination efforts, leaving B/Victoria as the dominant circulating strain.
• Generally Milder, But Serious: While often perceived as less severe than Influenza A, Influenza B can still cause significant illness, hospitalization, and death, particularly in children, adolescents, and older adults.
• Vaccine Match: The 2023-2024 quadrivalent flu vaccine includes a component for the circulating B/Victoria lineage. Vaccination remains the primary and most effective tool for prevention.
• Seasonal Timing: Influenza B often becomes more dominant later in the flu season, which typically peaks between December and February but can extend into May.

Background: The Virology of Influenza B

Understanding the Influenza Virus Family

Influenza viruses belong to the Orthomyxoviridae family. They are classified into four main types: A, B, C, and D. Human disease is caused primarily by types A and B. Influenza A is further subdivided into subtypes based on two surface proteins, hemagglutinin (H) and neuraminidase (N), such as H1N1 or H3N2. It has a vast reservoir in birds and other animals, allowing for frequent major changes (antigenic shift) that can cause pandemics.

Defining Influenza B: Structure and Lineages

Influenza B viruses are genetically more stable than Influenza A viruses. They lack an animal reservoir, circulating almost exclusively among humans, which limits their potential for dramatic antigenic shift. Instead, they undergo gradual antigenic drift. Influenza B is not divided into subtypes but into two distinct, co-circulating lineages:

• B/Victoria lineage: This is the lineage currently circulating globally. The 2023-2024 Northern Hemisphere vaccine contains a B/Victoria lineage virus (specifically, a B/Austria/1359417/2021-like virus).
• B/Yamagata lineage: This lineage has not been reported since March 2020. In October 2023, the World Health Organization (WHO) recommended removing the B/Yamagata component from future seasonal flu vaccines, as its circulation is believed to have ceased. This simplifies the vaccine back to a trivalent formulation for future seasons, though the 2023-2024 vaccine remains quadrivalent.

Analysis: Why is Influenza B on the Upward Push in 2023-2024?

The perception that Influenza B is “on the upward push” is not merely anecdotal; it is supported by robust national surveillance data. The CDC’s FluView Interactive weekly report has shown a consistent trend where the percentage of respiratory specimens testing positive for Influenza B has grown relative to Influenza A over the past month. Several interconnected factors likely contribute to this pattern:

1. The Natural Ebb and Flow of Flu Seasons

Influenza seasons do not follow a perfectly predictable script. The relative dominance of A and B strains can alternate from year to year and even within a single season. It is common for Influenza B to become more prominent in the latter half of the season, following an initial wave driven by an Influenza A (H3N2 or H1N1) strain. The current pattern aligns with historical observations where B strains drive significant outbreaks after the initial A peak.

2. Lower Recent Population Immunity to B/Victoria

Because the B/Yamagata lineage has been absent for years, and global circulation of B/Victoria was relatively low during the COVID-19 pandemic due to mitigation measures (masking, distancing), a larger cohort of children and even some adults may have limited prior exposure and immunity to the currently circulating B/Victoria viruses. This creates a susceptible population for the virus to exploit.

3. Vaccine Effectiveness and Uptake

Preliminary estimates for the 2023-2024 vaccine’s effectiveness against Influenza B are encouraging, with some studies suggesting moderate to good protection against medically attended illness. However, overall flu vaccination rates remain suboptimal. A population with lower vaccine coverage provides more opportunities for any flu virus, including B, to spread. The specific antigenic match between the vaccine strain and circulating B/Victoria viruses also plays a critical role in transmission dynamics.

4. Surveillance and Testing Patterns

Increased awareness and testing for influenza, especially in pediatric settings where Influenza B often has a pronounced impact, may contribute to the apparent rise. Laboratories are better at differentiating between A and B types now than in past decades, leading to more accurate and timely epidemiological data.

Practical Advice: Protection, Symptoms, and What To Do

Symptoms of Influenza B

Influenza B causes a classic acute respiratory illness. Symptoms typically begin abruptly and include:

• Fever or chills (not everyone with flu has a fever)
• Cough (usually dry)
• Sore throat
• Runny or stuffy nose
• Muscle or body aches
• Headaches
• Fatigue (can be severe and last 2-3 weeks)
• Some, especially children, may also experience vomiting and diarrhea.

It is impossible to distinguish between Influenza A and Influenza B based on symptoms alone; laboratory testing is required for definitive identification.

Prevention Strategies

Given the rising activity, reinforced preventive measures are crucial:

1. Annual Flu Vaccination: This is the single most important step. The flu vaccine is updated annually and is designed to protect against the viruses that surveillance indicates will be most common, including the circulating B/Victoria lineage. It is safe for everyone 6 months of age and older.
2. Practice Good Respiratory Hygiene: Cover coughs and sneezes with a tissue or your elbow. Wash hands frequently with soap and water for at least 20 seconds. Use an alcohol-based hand sanitizer if soap and water are not available.
3. Stay Home When Sick: Isolate yourself from others for at least 24 hours after your fever is gone (without the use of fever-reducing medicine) to prevent spreading the virus.
4. Consider Masks in High-Risk Settings: During periods of high community flu transmission, wearing a well-fitting mask in crowded indoor spaces can reduce transmission risk.
5. Clean and Disinfect: Regularly clean high-touch surfaces like doorknobs, phones, and keyboards.

Treatment and When to See a Doctor

Most people with the flu will recover with rest, fluids, and over-the-counter medications to manage fever and aches (e.g., acetaminophen or ibuprofen). However, certain individuals are at high risk for serious complications and should seek medical care promptly:

• Children younger than 2 years
• Adults 65 years and older
• Pregnant people
• People with chronic medical conditions (asthma, heart disease, diabetes, etc.)
• People with weakened immune systems

Antiviral medications (e.g., oseltamivir, zanamivir, baloxavir) can lessen symptoms, shorten the duration of illness by about one day, and prevent serious complications if started early—ideally within 48 hours of symptom onset. A healthcare provider can prescribe these if influenza is suspected or confirmed. Antibiotics are not effective against the flu virus but may be needed for secondary bacterial infections like pneumonia.

Frequently Asked Questions (FAQ)

Is Influenza B less dangerous than Influenza A?

Generally, Influenza A is associated with more severe seasonal epidemics and all historical pandemics. However, “less dangerous” does not mean “not dangerous.” Influenza B can and does cause severe disease, hospitalization, and death, particularly among children and older adults. The 2023-2024 season’s rising B activity underscores that it is a significant public health threat.

Can I get Influenza B if I already had Influenza A this season?

Yes. Infection with one type of influenza virus (A or B) does not provide immunity against the other type. It is possible, though uncommon in a single season, to be infected with both sequentially. This is one reason vaccination against both A and B components is so important.

Does the flu shot protect against Influenza B?

Yes. The standard quadrivalent flu vaccine protects against four strains: two Influenza A (H1N1 and H3N2) and two Influenza B lineages (from both the Victoria and Yamagata lineages). For the 2023-2024 season, it protects against the circulating B/Victoria lineage.

Why are children often more affected by Influenza B?

Studies have shown that Influenza B tends to have a greater impact on pediatric populations compared to Influenza A. This may be due to a combination of factors, including less pre-existing immunity in younger cohorts and specific viral characteristics that affect transmission and disease in school-aged children and adolescents.

If the B/Yamagata lineage is gone, why does the vaccine still have two B strains?

The 2023-2024 vaccine was manufactured months in advance based on WHO recommendations made in February 2023. At that time, while B/Yamagata circulation was very low, it had not been officially declared extinct. Therefore, the quadrivalent vaccine (with two B components) was produced. Future vaccines will likely revert to a trivalent format (one A/H1N1, one A/H3N2, one B/Victoria) following the WHO’s October 2023 recommendation.

Conclusion

The current “upward push” of Influenza B is a validated epidemiological trend, not an anomaly. It serves as a critical reminder that the flu season is dynamic and that both major types of influenza virus pose serious health risks. The dominance of the B/Victoria lineage this season highlights the importance of the quadrivalent flu vaccine, which offers specific protection against this strain. While the illness is often self-limiting, its potential for severe complications in vulnerable groups necessitates a proactive approach: get vaccinated, practice preventive hygiene, and seek antiviral treatment early if you develop flu-like symptoms and are in a high-risk group. Staying informed through official sources like the CDC and WHO is your best strategy for navigating the remainder of the flu season safely.

Sources

The information in this article is based on data and guidelines from the following authoritative public health and scientific institutions:

• Centers for Disease Control and Prevention (CDC). FluView Interactive. Weekly U.S. Influenza Surveillance Report. Accessed February 2024.
• World Health Organization (WHO). Recommended composition of influenza virus vaccines for use in the 2023-2024 northern hemisphere influenza season. February 2023.
• World Health Organization (WHO). Statement on the observance of the B/Yamagata lineage of influenza B virus. October 2023.
• Fleming, D., et al. (2023). Influenza B virus infections: Epidemiology, clinical disease, and vaccination. Influenza and Other Respiratory Viruses.
• U.S. Food & Drug Administration (FDA). Seasonal Influenza Vaccines. Information for Healthcare Professionals.