The traditional method for developing a vaccine uses substrates isolated from chicken eggs or mammalian cells. This method
has several drawbacks that could pose many problems. The first major issue is the time taken to develop a vaccine using this
method, which could be longer than six months. The nature of the disease agent can present significant challenges to cell-
and egg-based production depending on the species and strain of the agent. Moreover, there is a concern that people with
allergies to eggs may also react negatively to an egg-based vaccine. Using modern plasmid DNA-based vaccines to combat infectious
diseases is an alternative to traditional egg-based and mammalian-based vaccines. This article compares these two methods
of developing and producing vaccines.
The use of vaccines to fight infectious diseases is not a new practice. Vaccines have been around for over two hundred years
with one of the first documented being the vaccine for smallpox, which was developed and introduced in the 1790s. Over the
years, science and medicine have made great strides in developing and producing numerous successful vaccines to combat various
diseases. The smallpox vaccine developed by Jenner was prepared by taking scrapings of cowpox lesions directly from infected
cattle. The two polio vaccines developed in 1952 by Jonas Salk and Albert Savin used a revolutionary new technique that allowed
the propagation of polio virus in cultured monkey kidney cells.1 Today, effective vaccines for human diseases are produced by a variety of methods.
Developing Egg- and Cell-Based Vaccines
Traditional methods for developing and producing vaccines use substrates from either embryonated chicken eggs or mammalian
cells. The primary techniques for vaccine production used today are shown in Table 1. The table shows that chicken embryos
are primarily used for vaccines for viral diseases. The production of vaccines using fertilized eggs has over a fifty-year
history. Though this method has been successful in the fight against numerous diseases including influenza, measles–mumps–rubella
(MMR), and rabies, it has several drawbacks. A significant disadvantage is the time that it takes to develop a vaccine. Vaccines
that are developed using chicken eggs can take upwards of six to 12 months for development, production, and release testing
activities. This timeline would prove to be excessively long if a pandemic outbreak were to occur.5
Table 1. The primary techniques for vaccine production 4
Another concern regarding the production of vaccines in chicken eggs is that production capacity is limited to the number
and availability of specific pathogen-free fertilized eggs and the finite capacity of current manufacturing facilities. For
example, with the influenza vaccine, one egg is required for each dose of vaccine. Therefore, for a million doses, a million
chicken eggs must be processed. This method is not ideal for rapid large-scale production scale-up requirements. The specific
strain and species of the disease also can present challenges to the actual production of the vaccine in embryonated eggs.
Certain virus strains may not replicate productively in an embryonic avian host, which can then require additional process
optimization and production time. For example, the H5N1 strain of the avian influenza is generally deadly to chicken embryos
and therefore, an alternative method must be employed for the production of the vaccine.6 In addition, even for avian flu strains that could be produced in chicken embryos, in the event of a pandemic, the availability
of eggs would be severely impacted. Another disadvantage of producing vaccines in eggs is that people with allergies to eggs
may react negatively to egg-based vaccines.