If you buy hybrid egg layers at point of lay, they will have been raised commercially in very large flocks of chicks and growers, and will have been vaccinated against various chicken diseases at the early stages of their lives, just like children are. As Chuck says, this is to provide herd immunity to large flocks of birds, and is helpful for keepers of small flocks who want to avoid problems such as Mareks disease or mycoplasma in their birds.
If you buy birds from a small hobby breeder of purebred chickens, they may or may not have been vaccinated against anything, as the vaccination processes are gared more to very large numbers and the vaccines have to be used immediately once opened, so are expensive for just a few chicks. However, some private breeders do part of the full vaccination programme, such as the Mareks vaccine, which is helpful for this increasingly common and fatal virus,
There is an ongoing difference of opinion among chicken keepers about whether it is advisable to mix vaccinated and unvaccinated birds (usually this means keeping egglaying hybrids alongside purebreds from a different source.) Some people have had illness in their purebred flock as a result of this mixing because, it is claimed, the vaccinated birds have acted as carriers for infection they are themselves immune to. However, its probably fair to say that such problems are very rare and most people with mixed flocks have no problems at all. It's much more important to ensure the health of any new stock, as far as possible, and to quarantine new birds for two or three weeks at least, so there is less chance that anything they are carrying will transmit to your older birds. Introducing new birds from a different place is always risky to some extent. Even if they appear fully healthy, the process of moving to a new home is stressful to chickens and this means their immune systems can be lowered for a time, during which illness can take hold and could be spread to the existing flock. Hence the advisability of quarantine.
Vaccination at an early age has no effect on egglaying or meat, so long as the correct intervals between vaccination and production are observed. Since vaccines are given at an early age and long before the pullet comes into lay, there will be no problems with eating her eggs.
