Polyvalent influenza vaccine is a type of influenza vaccine that provides immunity against more than one type of antigen. In the second week after receiving the flu shot, the body's immune system is triggered by the antigens so the body starts producing antibodies. These antibodies help fight against influenza viruses. Influenza symptoms and deaths can be prevented by getting an influenza vaccine every year. Currently circulating influenza strains that can cause seasonal epidemics include influenza A viruses, which can be further divided into subtype A(H1N1) and A(H3N2), and influenza B viruses.
Influenza viruses were first identified in 1933 when a group of scientists, Wilson Smith, P.P. Laidlaw, and C.H. Andrewes, discovered that there was no bacteria infection from the throat samples of patients infected by influenza. They concluded that a virus caused the disease. After that, the researchers Thomas Francis and Jonas Salk developed the first influenza vaccine at the University of Michigan. The vaccine was tested for safety and effectiveness in the United States Army. It was afterward licensed for use. However, researchers had suspected that there was more than one viral strain of influenza as some patients did not develop antibodies to the strain discovered in 1933. By undergoing many investigations, the researchers discovered the influenza B virus. Thus, the influenza B virus was added to the vaccines and packaged as a bivalent vaccine in 1942.
There are multiple types of polyvalent influenza vaccines available in the market, including egg-based, cell-based, and recombinant influenza vaccines. The World Health Organisation (WHO) will announce the viral strains to be included in flu vaccines annually. Starting from 2021/2022, all influenza vaccines are quadrivalent, containing two influenza A and two influenza B antigens. These are called quadrivalent influenza vaccines (QIVs). Vaccination may also cause a range of adverse effects on patients, from fever, injection site pain to anaphylaxis, which is a severe allergic reaction.
Types of polyvalent influenza vaccines
There are a wide variety of influenza vaccines, classified by their types of different manufacturing processes.
Egg-based influenza vaccines
Egg-based influenza vaccines are produced by the oldest production process. This method has been used for over 70 years. First, the viruses are cloned into fertilised chicken eggs and incubated for a while. This usually last for several days, so the wanted viruses are allowed to replicate. The target viruses are then harvested from eggs, inactivated, or killed. They were then purified as vaccines.
The advantage of egg-based influenza vaccines is the low cost as the production does not require a sophisticated setup. However, the supply of these vaccines is dependent on the supply of eggs. Thus, the production process usually lasts for 6 to 8 months. Moreover, changes may occur while growing flu viruses in hen eggs. These are called egg-adapted changes that lead to differences in genes and structures between the candidate viruses in the eggs and the circulating, 'wild' viruses. As a result, less effective antibodies are made by the immune system of humans. Hence, egg-based vaccines may have lower efficacy in flu prevention.
Cell-based influenza vaccine
‘Cell-based’ vaccines referred to the vaccines that the virus strains are cultured in mammalian cells instead of in hen's eggs. Therefore, it can be used in people with egg allergies who are contraindiacted in having egg-based vaccination. The selected vaccine viruses are combined into mammalian cells. One possible type is Madin-Darby Canine Kidney (MDCK) cells. After allowing the viruses to reproduce for a couple of days, they are harvested, killed, and purified. In contrast to egg-based influenza vaccines, polyvalent influenza vaccines derived from mammalian cells are protected from egg-adaptation changes. As a result, the viruses cultured in mammalian cells will share more structural similarities with prevalent viruses. Theoretically, vaccines produced in cell culture may be more effective than vaccines produced in hen eggs. By making a comparison of the efficacy of cell-based and egg-based vaccines in a systematic review of English and Spanish-language studies, it was abundantly clear from the findings that there were no differences in the effectiveness of these two kinds of influenza vaccines. Another comparative study reflected that the efficacy of quadrivalent vaccines originated from cells was not superior to the egg-based quadrivalent vaccines among the elderly population aged ≥ 65 in the United States.