There are two main categories of viral vaccines—live (attenuated) and killed (inactivated) vaccines. A live attenuated vaccine is one where the virulence of the virus has been reduced, such that when the vaccine is administered to the patient it induces an immune response without causing clinical disease. The virus will replicate within the host and hence provide immunity for an extended period of time. The majority of successful viral vaccines fall under this category including vaccines for measles, mumps, rubella, influenza, yellow fever, and polio (Figure 1).
Inactivated viruses are used in cases where an appropriate attenuated vaccine has not been developed or in cases where the virus is thought to be likely to revert from the attenuated form into a more virulent form of the virus. The virus cannot replicate within the body and hence, there is typically a lower host response to the vaccine and often multiple doses are required. The most common inactivated viral vaccines include typhoid, rabies, and polio.
The development of viral vaccines requires viruses to be cultured in live cells, harvested, and then purified. Vaccine manufacturers are interested in monitoring the purity of the viral preparation at various key stages of the purification process and also in assessing the concentration of virus material present. This is where the NanoSight instrument (Figure 2) adds real value. The particle-by-particle approach to sizing and counting viruses can distinguish viruses from larger cell debris, and high-resolution number distributions can be used to calculate the number of viruses vs. the number of virus aggregates.
Estimating the concentration of viruses present is essential in understanding the loss of product at each step of the purification process. This information can be used to optimize the process in terms of product yield. Virus concentration is also essential when trying to understand dosage in the final product.