2018 marks 100 years since the 1918 influenza pandemic, also known as the Spanish Flu. In merely two years, this plague swept across five continents, infected over 500 million people, and killed over 50 million1. Now, 100 years later, scientists are still striving to understand how to prevent future influenza outbreaks. Despite the tremendous progress of medicine, influenza remains a considerable threat to public health. According to the Center for Disease Control (CDC), influenza affects 5 to 20 percent of the population and kills over ten thousand people annually in the US alone. This disease also causes a great economic burden by generating over $16 billions of annual direct medical expenses2. The 2017-2018 winter flu season has been particularly severe, with over 26,000 hospitalizations and 133 pediatric deaths as of March 173. Fortunately, a recent study published in Science may have brought us one step closer to conquer- ing influenza4. Led by Sun Ren, a team of scientists from the US and China developed a rationally designed mutant influenza virus with the potential application as a more effective flu vaccine.

According to recommendations by the World Health Organization (WHO), vaccination is the best way to prevent influenza5. However, the efficacy of influenza vaccine is disappointing. Annual effectiveness of the influenza vaccine between 2004 and 2017 varied between 10 and 60 percent, dramatically lower than that of vaccines against chickenpox (94%) or mumps (88%) 6,7,8. The low efficacy is largely due to the highly variable nature of influenza viruses. Current flu vaccines contain three or four strains of influenza antigens, which are inactivated (dead) or attenuated (weakened) viruses. After injection, the antigens prime B cells, a type of immune cell, to recognize and defend against their corresponding viruses by secreting antibodies. Antibodies are highly specific molecules that bind to unique antigens and tag them for destruction by other immune cells. However, because influenza viruses mutate quickly, they contain a diverse and constantly changing range of antigens, many of which cannot be targeted by a single vaccination. Many scientists are working on developing a vaccine whose strength is “just right”: strong enough to the prime the immune system to defend against diverse viral strains, but weak enough to avoid causing influenza symptoms. With this goal in mind, Sun and colleagues took a novel genetic approach to vaccine development. By investigating the functions of influenza genes, they believed it is possible to identify a mutant virus that can induce strong immunity without causing disease.










Driven by natural selection, influenza has evolved genes responsible for deactivating the immune system’s antiviral defense mechanisms. In particular, influenza viruses suppress the function of type I interferon (IFN), signaling molecules that can trigger long-lasting but controlled antiviral immune responses such as B and T cell development and dendritic cell maturation9. Based on previous studies10, Sun and colleagues hypothesized that mutating the IFN-suppressing genes may generate a weakened virus suitable for a vaccine. To find these genes, they created over 10,000 mutants of a H1N1 influenza virus, each with a single base pair in their genome mutated. They then transfected the mutants into cell lines cultured with or without IFN and quantified viral abundance through gene sequencing. Sequencing data suggested that some mutants grew normally without IFN but did not grow in the presence of IFN, indicating that these mutations caused sensitivity to IFN. To maximize sensitivity to IFN in the influenza mutants, the researchers then created a hyper-interferon-sensitivity (HIS) virus with all eight mutations found in the most IFN-sensitive mutants. When various cell lines were transfected with HIS virus, they indeed showed higher IFN- induced antiviral activities compared to cell lines transfected with the unmutated virus. Notably, the HIS virus-transfected cells did not produce signaling molecules that can cause inflammation and flu symptoms. To further test the safety and efficacy of the HIS virus as a vaccine, the authors administered mice and ferrets with either the HIS virus or an existing influenza vaccine of attenuated viruses, and subsequently infected them with active influenza viruses. After vaccination, no animal displayed signs of morbidity such as weight loss or rapid breathing even for the highest dosage tested. The HIS virus induced B cells to produce more antibodies than the existing vaccine, and the antibodies produced covered a broader range of different viral antigens. Even more excitingly, the HIS virus elicited various types of T cell responses, which confer longer and broader protection than antibodies. This feature represents a major advantage over existing flu vaccines, which activate B cells but not T cells. Finally, the authors showed that animals vaccinated with the HIS virus were protected against four different influenza strains even 28 days after vaccination. Overall, the HIS virus triggered robust immune activation without causing any influenza symptoms, making it a promising candidate as a vaccine.










Is the HIS virus the secret recipe for a more effective flu vac- cine? Before scientists explore its therapeutic potential, several key concerns remain. First, the full breadth of protection offered by this vaccine has yet to be tested. Although Sun and colleagues’ work demonstrated its efficacy against four influenza strains, the vaccine might not be effective against other strains. Second, using a live virus as a vaccine requires rigorous examination to ensure its safety for immunocompromised individuals. Third, the vaccine needs to be tested for longer than 28 days to ensure that it offers long-term protection. Despite these remaining questions, this work offers an exciting and innovative new path towards developing a better vaccine and saving thousands of lives each year. The rational, genetic-based workflow used in this study may also be valuable for developing vaccines against other diseases.