The first human vaccines against viruses were based using weaker or attenuated viruses to generate immunity. The smallpox vaccine used cowpox, a poxvirus that was similar enough to smallpox to protect against it but usually didn’t cause serious illness. Rabies was the first virus attenuated in a lab to create a vaccine for humans.
Egg-Based Vaccines
Over the last 60 years, seasonal flu vaccines have been manufactured using fertilized embryonic eggs. 
Using this method, it takes about four months to produce a batch of vaccines for a new strain of influenza virus; from the moment the new influenza virus’ culture becomes available for vaccine manufacturing. The advantages of using embryonic eggs to manufacture seasonal flu vaccines are that the safety and effectiveness of the vaccines produced have been well established.
Cell-Based Vaccines
Since the mid 1990’s, newer vaccine manufacturing methods were developed. The cell-based vaccine manufacturing process is one of such methods. The cell-based vaccine manufacturing process uses cells from mammals to culture the influenza virus for vaccine production. Various pharmaceutical companies use different sources of mammalian cell cultures for the vaccine manufacturing process. Baxter Healthcare uses cells extracted from the kidney of the African Green Monkey while companies such as Solvay Biologicals and Novartis Vaccines use kidney cells from canines to produce seasonal flu vaccines.
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