Final answer:
d. Both a and c
Killed vaccines have a disadvantage of providing shorter-duration immunity and can lead to more side effects due to the need for higher doses and boosters, unlike live vaccines which offer a longer-lasting immune response.
Step-by-step explanation:
Among the disadvantages of killed vaccines compared to live vaccines are that they provide a shorter duration of immunity and can cause more side effects.
Killed vaccines usually require higher doses and multiple boosters to maintain immunity, leading potentially to inflammatory reactions at the injection site. This is in contrast to live vaccines which generally induce a more comprehensive and longer-lasting immune response. It's important to note, however, that killed vaccines do not carry the risk of causing severe active infections, as they contain pathogens that are inactive or dead and cannot replicate within the host.