Researchers at the University of Pennsylvania Perelman School of Medicine have developed an experimental multivalent mRNA-based vaccine against all 20 known influenza virus subtypes. Their approach differs from previous attempts to make a universal flu vaccine by including specific antigens for each subtype, rather than just a smaller set of antigens shared between subtypes. This strategy leverages the same mRNA technology used in the Pfizer and Moderna SARS-CoV-2 vaccines. The mRNA technology that enabled those COVID-19 vaccines was pioneered at Penn.
Tests in animal models showed that the vaccine dramatically reduced signs of illness and protected against death, even when the animals were exposed to different flu strains than those used to make the vaccine.
The team suggests that their technology could lead to the development of a universal flu vaccine that protects against possible future pandemics. “The idea here is to have a vaccine that gives people a basic level of immune memory for various strains of flu, so that there is much less illness and death when the next flu pandemic hits,” said the study’s lead author. , Scott Hensley, PhD. , professor of microbiology at the Perelman School of Medicine. Hensley and his colleagues reported the development of their mRNA vaccine in Sciences, in a report titled “A multivalent nucleoside-modified mRNA vaccine against all known influenza virus subtypes.” Hensley and her lab collaborated on the study with the lab of mRNA vaccine pioneer Drew Weissman, MD, PhD, Roberts Family Professor of Vaccine Research and Director of Vaccine Research at Penn Medicine.
Influenza viruses periodically cause pandemics with enormous numbers of deaths. The best known of them was that of 1918.–19 “Spanish flu” pandemic, which killed tens of millions of people worldwide. Influenza viruses can circulate in birds, pigs and other animals, and pandemics can start when one of these strains jumps to humans and acquires mutations that make it better suited to spread between humans. The authors explained: “There are at least 18 different influenza A virus (IAV) subtypes circulating in animal reservoirs, and these viruses occasionally enter the human population and cause a pandemic.”
Current flu vaccines are “seasonal” vaccines that protect against recently circulating strains, but are not expected to protect against new pandemic strains. And even with increased global surveillance, it’s hard to predict which flu strain will cause the next flu pandemic, making a universal vaccine important. As the researchers noted, “although surveillance programs and modeling studies have increased our knowledge of pandemic risk, we cannot accurately predict which influenza subtype will cause the next pandemic.”
There are several universal influenza vaccines in development to provide protection against various subtypes of the influenza virus, the team continued. Most of these vaccine candidates include a limited number of antigens that have epitopes that are conserved across different influenza virus subtypes.
Rather, the strategy employed by the Penn Medicine researchers is to vaccinate with immunogens, a type of antigen that stimulates immune responses, of all known influenza A and B virus (IBV) subtypes to obtain broad protection. This vaccination strategy is not expected to provide “sterilizing” immunity that completely prevents viral infections. “Instead of focusing on immunogens to generate antibodies against epitopes that are conserved among many different influenza virus strains, we designed a vaccine that encodes separate immunogens from all known IAV subtypes and IBV lineages,” the team explained.
Their recently reported study confirmed that the vaccine elicited a memory immune response that can rapidly recover and adapt to new pandemic viral strains, significantly reducing severe illness and death from infections. “It would be comparable to the first-generation SARS-CoV-2 mRNA vaccines, which were directed at the original Wuhan coronavirus strain,” Hensley said. “Unlike later variants like Omicron, these original vaccines did not completely block viral infections, but continue to provide long-lasting protection against severe disease and death.”
For their flu vaccine, the researchers prepared 20 different nanoparticle-encapsulated mRNAs, each encoding a different hemagglutinin antigen. The experimental mRNA lipid nanoparticle vaccine developed by Hensley and his colleagues encoded hemagglutinin (HA) antigens from all 20 known influenza A and B virus subtypes.
When injected into and absorbed by recipient cells, the vaccine resulted in the production of copies of the key influenza virus hemagglutinin protein, for all 20 influenza hemagglutinin subtypes: H1 through H18 for influenza viruses. influenza A and two more for influenza B viruses. “For a conventional vaccine, immunization against all of these subtypes would be very challenging, but with mRNA technology, it’s relatively easy,” Hensley said.
Tested in mice, the mRNA vaccine elicited high levels of antibodies, which remained elevated for at least four months, and reacted strongly to all 20 influenza subtypes. Multivalent protein vaccines produced using more traditional methods elicited fewer antibodies and were less protective compared to the multivalent mRNA vaccine in animals.
In addition, the new vaccine appeared relatively unaffected by previous influenza virus exposures, which can skew immune responses to conventional influenza vaccines. The researchers observed that the antibody response in the mice was strong and widespread, regardless of whether the animals had been exposed to the flu virus before or not. The team also carried out tests on ferrets that were vaccinated using a boost approach and challenged with an avian H1N1 virus, to mimic a pandemic caused by an unknown viral strain. The results of these experiments confirmed that, compared to unvaccinated animals challenged by the same virus, the vaccinated ferrets lost less weight and all survived, while two of the four unvaccinated ferrets died. Unvaccinated animals also showed more clinical signs of disease relative to vaccinated animals after infection.
“Further studies will be required to fully elucidate the mechanisms by which the 20-HA mRNA vaccine provides protection,” the authors acknowledged. Their reported findings suggested that protection against antigenically matched strains is mediated by neutralizing antibodies, whereas protection against mismatched viral strains may occur through non-neutralizing mechanisms, such as antibody-dependent cellular cytotoxicity (ADCC).
Hensley and his colleagues are currently designing human clinical trials. The researchers envision that if these trials are successful, the vaccine may be useful in gaining long-term immune memory against all influenza subtypes in people of all ages, including young children. “We believe that this vaccine could significantly reduce the chances of getting a serious flu infection,” Hensley said. As the team noted in their report, “mRNA influenza vaccines that combine imperfectly with new pandemic influenza virus strains are likely not to provide sterilizing immunity, but rather to limit the severity of illness and protect against death through non-neutralizing mechanisms”.
In a complementary perspective, Alyson A Kelvin, PhD, and Darryl Fallzarano, PhD, of the University of Saskatchewan, noted that strengths of the mRNA platform for pandemic vaccine production include flexibility of antigen design, a greater number of potential viral targets, the speed of production, and economical and scalable manufacturing. “These strengths are important when designing and producing vaccines for a highly diverse and unpredictable family of viruses that can easily spread globally in a matter of weeks,” they noted. However, they commented, “questions remain about the regulatory and approval pathway for such a vaccine that targets viruses with pandemic potential but not currently in human circulation.”
In principle, Hensley added, the same multivalent mRNA strategy could be used for other viruses with pandemic potential, including coronaviruses. “Multivalent mRNA-LNP vaccines can be applied against other variable pathogens, such as coronaviruses and rhinoviruses,” the scientists concluded. “For example, SARS-CoV-2mRNA vaccines are being updated to include multiple spike components to combat antigenically distinct strains. Further studies will be required to determine the maximum number of antigens that can be delivered simultaneously via mRNA-LNP vaccines and the underlying immunological mechanisms that allow the induction of responses against multiple antigens.”