Why do we need an annual flu shot? There are 2 main reasons why we need seasonal influenza vaccinations:
Strains of influenza change annually
Flu vaccine efficacy is narrow and short-lived
The flu is primarily caused by the influenza A virus (IAV), an enveloped RNA virus that has several different strains. Its membrane contains proteins known as Hemagglutinin (HA) and Neuraminidase (NA) which are important for entry and release of the virus. Other structural components of the virus, such as the ion channel M2 protein, can be recognized by our immune systems. While our immune system can mount a response to an influenza infection, this type of response is usually specific to the strain of influenza causing infection. Most neutralizing antibodies generated by either infection or vaccination target what is known as the globular head of HA. As shown in the graphic below, HA and NA are more numerous and accessible on the viral envelope and are therefore more accessible antibody targets than the M2 protein.
The viral membrane contains several membrane proteins, such as hemagglutinin (HA) and neuraminidase (NA).
Strains of influenza change annually. The reason we need yearly flu vaccinations is partially due to changes in the sequence of the HA protein. A person exposed to one strain of IAV will develop neutralizing antibodies specific to the HA globular head of that strain, which bind to the HA and 'neutralize' its ability to attach to its cellular ligand. Random mutations in IAV lead to variability in the globular head of HA over time, a process known as antigenic drift. Antibodies that recognize a previous strain will no longer protect against the new variant. Recombination of 2 different strains of viruses in the same infected host can yield a completely new HA that has never been seen by our immune system, a process known as antigenic shift. Such antigenic shifts have caused pandemic strains of influenza such as the 2009 H1N1 outbreak.
Flu vaccine efficacy is narrow and short-lived. Due to this variation in the virus, most influenza vaccinations have short-lived efficacy and narrow protection. Efficacy, in this scenario, refers to broad protection against multiple strains of influenza. Our immune system is great at generating strain-specific antibodies. This leads to a phenomenon originally named the “original antigenic sin” by Thomas Francis: previous exposure to an initial influenza strain leaves an immunological memory. When a new strain infects us, there is competition between the memory B cells, cells that produce antibodies towards specific targets due to past exposure, that are activated by the infection and naive B cells, cells that have the capacity to generate and improve antibodies in response to a new infection, that are responding to the new strain. This causes a greater threshold for activation that has to push over the hump of the old memory to make antibodies that are specific to the new strain.
New seasonal vaccinations must be developed to provide protection against strains predicted to be common in the upcoming flu season. These responses are short-term because the viruses themselves change: while we may be protected from this year’s virus X with the current vaccine, the antibodies made for virus X will not protect against next year’s virus Y. This is why the CDC analyzes the data on circulating strains to predict what strains to vaccinate against in the coming year. This predictive nature of vaccine design is one of the primary reasons why vaccine efficacy varies widely from year to year.
Adjuvants and Vaccine Efficacy.Vaccines have 2 main components: antigens and adjuvants. In flu vaccines, the antigen is an inactivated virus that entices our immune cells to make protective antibodies against this viral strain. Adjuvants help immune cells efficiently respond to the antigen. For years, the standard formulation of vaccines have used aluminum based adjuvants such as alum. Over 146 approved vaccines contain aluminum-based adjuvants, making these the most common option. While great at generating a strong immune response, these adjuvants are not able to stimulate cell-mediated immunity, which often leads to lower efficacy in vaccines.
Oil-in-water emulsions are another class of adjuvants. These adjuvants use lipids and non-polar compounds to stimulate the immune system to generate antibodies towards a target antigen. These adjuvants have been used for influenza vaccinations in the past, and newer emulsions are undergoing clinical trials. Unfortunately, this type of adjuvant has been noted to cause more adverse side effects at the site of vaccination, such as pain, and bruising.
One of the newest approaches towards adjuvants involves using bacterial components, such as flagellin or modified Lipid A from LPS, to trigger a more robust immune response. Flagellin triggers TLR5, which ultimately leads to activation of dendritic cells, T cells, and B cells. The addition of flagellin components to a vaccine is thought by researchers to provide a better immune response than alum or oil-in water emulsion adjuvants. The goal for these newer adjuvants is clear, to increase vaccine efficacy while providing a balanced immune response. Thus scientists continue to search for new adjuvants that will further increase vaccine performance in a safe and effective manner.
Strategies For A Universal Vaccine
The variability of the globular head of HA and the immune system’s preference for these epitopes, the antibody target, has led to the search for more conserved epitopes across multiple influenza strains. If vaccines can focus the immune response against the regions of IAV that undergo less mutation, there would be a greater probability of protection on a near-universal level against strains of IAV. Two of these strategies center on conserved regions found within HA: recombinant stalk-specific HA and chimeric recombinant HA.
Strategy
Pros
Cons
Recombinant stalk-specific HA
-Simple, straightforward approach -Generates antibodies toward a conserved region in influenza
-Maintains native HA structure -May provide protection against future pandemic strains -Can use any vaccine development platform
-Does not enrich for stalk-specific antibodies
Recombinant M2
-Conserved structure in influenza -M2’s function as an ion channel makes it harder for influenza to mutate
-M2 is less accessible to antibodies compared to HA or NA
Recombinant stalk-specific HA. The stalk, which is the domain of HA that anchors the globular head to the membrane of the virus, is relatively similar across IAV strains. One strategy to exploit this involves a recombinant HA protein that lacks the globular head and contains only the stalk domain. To date, most vaccine development using this approach remains in the preclinical stage of research.
Chimeric recombinant HA. The second strategy toward a universal vaccine uses reverse genetics to make viruses expressing recombinant chimeric HA proteins. These constructs typically have the same stalk (the H1 clade of widely circulating IAV strains) fused with the globular head of non-human IAV strains. Sequential vaccination against these chimeric HAs that share the same stalk aims to generate stalk-specific antibodies that may provide universal protection against IAV.
GlaxoSmithKline (GSK) started clinical trials of 2 different chimeric HA based vaccines. The first was a chimeric HA antigen with a novel adjuvant and began clinical trials in 2017. This vaccine completed Phase I, which tests the safety of multiple doses on healthy volunteers, on May 31st of 2019. Initial plans by GSK had the vaccine moving into Phase II, which tests the efficacy and potential side effects. However, in May of 2019, GSK ended development of this vaccine. This decision came after early analysis of Phase I data revealed that while the vaccine provided cross-protection, the boosting effect of multiple doses was not as strong as expected. Additionally, the data revealed that the vaccine would likely not provide the sought after long-term (i.e longer than a flu season) protection. GSK has another chimeric HA vaccine in clinical trials. A second, collaborative vaccine between GSK, Icahn School of Medicine at Mount Sinai, and Duke University is currently in Phase I.
This chimeric HA approach means this vaccine has the potential for protection against novel pandemic IAV strains, giving it an advantage over the headless stalk approach. Since non-human influenza HA globular heads were used, our immune system will generate strain-specific antibodies to the head along with the conserved stalk domain. Therefore, if a pandemic strain ever expressed that same HA, people would be protected from an otherwise potentially lethal infection.
While many IAV researchers still believe that stalk-specific antibodies will ultimately be the most protective strategy, using this chimeric approach allows the immune system to develop antibodies against the non-human HA globular heads. Whether these will actually confer protection against novel IAV strains remains to be seen.
Recombinant M2 and other Non-HA Vaccine Strategies. Other conserved regions in IAV include the exposed surface domain of the M2 protein, another structural protein found in the virus. The M2 protein has not been a major target previously due to the lack of accessibility to antibodies. HA and NA specific antibodies are more common than M2 specific antibodies. VaxInnate currently has Phase I and Phase II trials for VAX102, a recombinant protein that links 4 copies of the exposed M2 domain to the flagellin of S. typhimurium. This strategy hopes to generate neutralizing antibodies towards the conserved M2 protein while using flagellin as a TLR5 adjuvant.
Bioinformatics analysis of conserved peptides across IAV strains offers another approach. Biondvax’s M-001 and Immune Targeting Systems’ FP-01.1 are both vaccines created using this method and are undergoing clinical trials. M-001 has reached Phase III, which tests the vaccine for efficacy in a clinical setting, while FP-01.1 is in Phase I.
How “Universal” Are Universal Influenza Vaccines?
While many of the above strategies protect against a variety of IAV strains, researchers do not yet know how protective these vaccines will be in people. For example, dosage effects of the chimeric HA vaccines differ in mouse model experiments compared to preliminary results from human trials. Mice require at least 3 immunizations with the chimeric HA antigen to achieve sufficient levels of anti-stalk neutralizing antibodies, whereas humans require only 2 immunizations (20). Mouse models can never fully capture the immunological history of people who experience both bouts of flu and receive influenza vaccinations. Therefore, human trials are critical to test vaccine performance in people.
There is also a concern regarding the length of protection conferred by a universal vaccine. Most seasonal influenza vaccinations only provide a short-term period of efficacy, which is a second reason we need to get vaccinated every year. While the efficacy of several universal vaccine candidates are being assessed, it appears that many of these vaccines may also only provide short-term efficacy. Thus, instead of being a one-time replacement to the seasonal vaccine, these new strategies may only serve to replace it with a more efficacious vaccine.
It is clear that while significant progress has been made to develop a broadly protective universal vaccine, there is still more work to be done to achieve a long-term solution to influenza. While influenza continues to strike year after year, promising work on broadly effective vaccines may ultimately break our never-ending cycle of annual influenza vaccinations.
