Virus (from the Latin virus meaning “toxin” or “poison”) is a microscopic infectious agent that can reproduce only inside a host cell. Viruses consist of two parts: nucleic acid and capsid. Some viruses have a viral envelope. The diameter of most viruses is between 10 and 300 nm.
Viruses are generally attenuated via passage—growing several times in unrelated or foreign hosts such as tissue culture, embryonated eggs, or live animals. Likely, one of these will possess a mutation that enables the virus to infect the new host. However, this mutant normally has a lower virulence than the virus that was in the original host. The genetic information for interacting with the host does not change, enabling it to infect the host, but it causes less damage and so acts as a vaccine. Some of the modern vaccines use genetic engineering to precisely induce attenuation by selective mutation, gene deletion, or substitution. Examples are dengue vaccine and Japanese encephalitis (JE) vaccine.
Attenuated vaccines offer quick immunity, activate all phases of the immune system, and provide more durable long-term immunity. However, secondary mutation can cause a reversion to virulence. This means the vaccine may be able to cause disease in immunocompromised patients (those with AIDS, for example). Additionally, they can be difficult to transport because they must be maintained under certain conditions, such as temperature, to guarantee the survival of the virus.
The live attenuated viral vaccine manufacturing follows a complex, multi-step process. It is not a templated process. The manufacturing process for each viral vaccine is different and is dictated by shape, size, nature, physico-chemical behavior, stability, and host specificity. Though different manufacturers follow different process flows, a general outline of the process is summarized in Figure 2. An important manufacturing challenge is to keep the attenuated virus live and maintain the infective potential of the viral vaccine throughout the downstream processing and formulation until it is administered to healthy individuals. The end objective is to elicit sufficient protective immune response (neutralizing type antibody) against the designated virus upon immunization.
In this paper we review the steps in the viral vaccine manufacturing process.