Madison Clarke
The effectiveness of the flu vaccine varies by influenza virus type or subtype and by vaccine. Seasonal flu vaccines are designed to protect against the influenza viruses expected to be most common during the upcoming flu season. The H3N2 virus is a subtype of influenza A, and vaccines against influenza A(H3N2) viruses tend to be less effective than vaccines against other influenza viruses due to frequent antigenic changes. While the composition of flu vaccines is reviewed annually, it is still possible for antigenic changes to occur between the virus components of the vaccine and the circulating influenza viruses.
| Characteristics | Values |
|---|---|
| Effectiveness of H3N2 vaccine | 22% pooled VE estimate for the Northern Hemisphere |
| Effectiveness against other viruses | Better protection against influenza B and influenza A(H1N1) viruses |
| Effect on illness, hospitalization, and death | Vaccination can still prevent illness, hospitalization, and death |
| Recommended protocol | Two doses two to four weeks apart, with revaccination within one year |
| Annual flu vaccine recommendation | Everyone 6 months and older should get an annual flu vaccine |
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What You'll Learn
H3N2 vaccines for dogs
Canine influenza, or dog flu, is a highly contagious respiratory disease caused by two specific type A influenza viruses known to infect dogs: the H3N8 virus and the H3N2 virus. Most dogs in North America have no pre-existing immunity to canine influenza, making them highly susceptible to infection. The majority of canine influenza cases have been associated with multi-centric, non-sustaining outbreaks.
The H3N2 virus spread to dogs around 2006 and has since posed a serious threat to the health and safety of dogs. Dogs infected with the H3N2 virus may start showing respiratory signs between two and eight days after infection. Infections are most contagious during the incubation period.
The Nobivac® NXT Canine Flu H3N2 vaccine is a groundbreaking new way to safeguard dogs against the ongoing threat of canine flu. It uses RNA particle technology to stimulate the immune system to attack the H3N2 virus without the use of adjuvants or live organisms. A low-volume 0.5 mL dose provides a more gentle vaccine experience.
Initial vaccination can begin as early as six weeks of age, with the recommended protocol being two doses two to four weeks apart, followed by revaccination within one year of the second dose. Subsequent boosters are then given annually. A single vaccination is unlikely to be protective in high-risk settings.
In addition to Nobivac, the VLP influenza vaccine is another promising alternative to the traditional inactivated vaccine. The VLP vaccine has been shown to induce a comprehensive immune response and provide cross-protection against the challenge of homologous and heterologous influenza viruses through various routes of administration.
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Effectiveness against H3N2 viruses
The effectiveness of the flu vaccine may vary depending on the type or subtype of influenza virus, and how similar the virus is to the circulating influenza viruses. Flu vaccines protect against influenza viruses but do not protect against other viruses that cause flu-like symptoms. Since 2009, studies have shown better protection against influenza B and influenza A(H1N1) viruses than against influenza A(H3N2) viruses. A meta-analysis of 76 VE studies found that flu vaccines were least effective against influenza A(H3N2) viruses. The pooled VE estimate for the Northern Hemisphere was 22%, compared to 56% for influenza A(H1N1)pdm09 viruses and 42% for influenza B viruses.
There are several reasons why flu vaccine effectiveness against influenza A(H3N2) viruses may be lower compared to other influenza viruses. Influenza A(H3N2) viruses undergo more frequent changes that result in differences between the virus components of the flu vaccine and circulating influenza viruses (antigenic changes). These changes can reduce the potential effectiveness of the vaccine.
The composition of flu vaccines is reviewed and updated annually. Trivalent flu vaccines, which were used in the United States from 1978-1979 to 2012-2013, included components for one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B virus. Quadrivalent flu vaccines, which were introduced in the United States during the 2013-2014 flu season, added a second influenza B virus to protect against both lineages of influenza B viruses (B/Yamagata and B/Victoria). However, due to the absence of detected influenza B/Yamagata lineage viruses since March 2020, the World Health Organization (WHO) and FDA recommended removing this component from flu vaccines. As a result, only trivalent flu vaccines will be available in the United States for the 2024-2025 flu season.
While flu vaccines can provide protection against different influenza viruses, their effectiveness may vary. The influenza A(H3N2) virus has been associated with lower vaccine effectiveness due to frequent antigenic changes. The composition of flu vaccines is regularly reviewed and updated to improve protection against circulating influenza viruses.
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Egg-adapted changes
Seasonal flu vaccines are designed to protect against the influenza viruses expected to be most common during the upcoming flu season. However, the effectiveness of these vaccines varies based on how similar the vaccine virus is to the circulating influenza viruses. The amount of protection provided may also differ by influenza virus type or subtype and by vaccine.
The H3N2 virus is known to evolve rapidly, leading to poor vaccine efficacy. The substitutions employed during vaccine production using embryonated eggs contribute to this poor efficacy. This process, known as egg passage adaptation, involves the virus acquiring mutations on the hemagglutinin (HA) protein to facilitate its growth in eggs. These "egg-adapted changes" or "egg-adaptive mutations" can alter the antigenic properties of HA, reducing vaccine effectiveness.
The preference for specific egg-adaptive mutations varies across different strains and clades of the H3N2 virus. For example, recent studies have identified two prevalent natural variants, Thr160 and Asn190, which prevent the emergence of the L194P mutation, known to significantly impact antigenicity. Other mutations, such as H183L, G186V, and A196T, have been found to have minimal effects on antigenicity.
While egg-adaptive mutations are critical for high-yield production in egg-based vaccines, they can negatively impact vaccine effectiveness. Therefore, it is essential to select optimal vaccine viruses and predict egg passage substitutions to minimize the impact of these mutations on antigenicity. Advanced techniques like machine learning models can aid in this selection process, improving the effectiveness of seasonal influenza vaccines.
To address the limitations of egg-based vaccines, alternative vaccine production methods, such as cell-based and recombinant flu vaccines, are being explored. These methods do not rely on eggs, avoiding the issue of egg-adaptive changes and improving protection against circulating influenza viruses.
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Flu vaccine types
Seasonal flu vaccines are designed to protect against the influenza viruses expected to be most common during the upcoming flu season. The flu vaccine effectiveness may vary based on how similar the vaccine virus is to the circulating influenza viruses. However, even when the viruses are very different, vaccination can still prevent illness, hospitalisation, and death. Flu vaccines do not protect against other viruses that cause flu-like symptoms, such as respiratory syncytial virus (RSV).
There are several types of flu vaccines offered at many convenient locations, including physician offices, public health departments, pharmacies, retail stores, workplaces, and schools. Many insurance plans cover annual flu vaccination.
The most commonly administered flu vaccine is a quadrivalent vaccine, which provides protection against the four strains of influenza most likely to be circulating each season. These vaccines are administered intranasally and are known as "live attenuated" because they include minuscule bits of weakened, live viruses that are modified to create an immune response without causing flu illness. Live attenuated vaccines are approved for use in individuals aged 2 to 49 years.
Another type of flu vaccine is the recombinant flu vaccine, which is produced without the use of eggs. This vaccine contains three times the antigen compared to other standard-dose inactivated flu vaccines.
Cell-based vaccines are also produced without eggs, using animal cells in liquid culture instead. This type of vaccine is approved for people aged 6 months and older.
The inactivated influenza vaccine is preferentially recommended for adults aged 65 years and older. This vaccine is administered as an intramuscular injection.
The H3N2 virus-like particle (VLP) vaccine is another example of a vaccine that has been shown to provide sterilizing protection against homologous H3N2 canine influenza virus.
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Annual flu vaccines
Seasonal flu vaccines are designed to protect against the influenza viruses expected to be most common during the upcoming flu season. The composition of flu vaccines is reviewed and updated annually. Vaccination can prevent illness, hospitalisation, and death, even when the viruses are very different from those targeted by the vaccine.
Flu vaccines are available in two types: trivalent and quadrivalent. Trivalent vaccines protect against three different influenza viruses: one influenza A(H1N1) virus, one influenza A(H3N2) virus, and one influenza B virus. Quadrivalent vaccines protect against four viruses: one influenza A(H1N1) virus, one influenza A(H3N2) virus, one influenza B/Victoria lineage virus, and one B/Yamagata lineage virus.
The effectiveness of flu vaccines may vary depending on the similarity between the vaccine virus and circulating influenza viruses. Since 2009, studies have indicated better protection against influenza B and influenza A(H1N1) viruses compared to influenza A(H3N2) viruses. The lower effectiveness against influenza A(H3N2) viruses may be due to the frequent antigenic changes that occur in these viruses, resulting in differences between the vaccine and circulating virus components.
It is recommended that individuals aged six months and older receive an annual flu vaccine to protect against the most common influenza viruses expected during the flu season. However, it is important to note that flu vaccines do not protect against all flu-like illnesses caused by other viruses. Additionally, a single flu vaccine may not provide protection against newer strains of the influenza virus that may emerge.
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Frequently asked questions
No, the protection provided by a flu vaccine may vary by influenza virus type or subtype. The H3N2 virus is known to undergo frequent changes, which can reduce the effectiveness of vaccines.
The pooled VE estimate for the H3N2 vaccine in the Northern Hemisphere was 22%.
Seasonal flu vaccines are designed to protect against the influenza viruses expected to be most common during the upcoming flu season. The H3N2 vaccine includes viral proteins from the H3N2 virus to induce an immune response.
Yes, other types of flu vaccines such as cell-based and recombinant flu vaccines are produced without the use of eggs, so the viruses used do not undergo the same changes as those in the H3N2 vaccine.
