Varicella vaccine is very safe. About 20% of vaccine recipients will have minor injection site complaints, such as pain, swelling, or redness. Fewer than 5% of recipients develop a localized or generalized varicella-like rash 5 to 26 days after vaccination. These rashes have an average of 2 to 5 lesions, and may be maculopapular rather than vesicular. Fever following varicella vaccine is uncommon.
Ask the Experts: Varicella (Chickenpox): Vaccine Safety
If you believe the child had varicella disease (that is, breakthrough varicella) after the first dose, the child does not need another dose. If you are uncertain whether the child had varicella or a rash related to varicella vaccination, the second dose should be administered on schedule. If in doubt, give the second dose. If this was a case of breakthrough varicella, a second dose will not be harmful.
Transmission of varicella vaccine virus is a rare event, and appears to occur only when the vaccinated person develops a vesicular rash. A maculopapular rash 2 weeks after varicella vaccine may not have been caused by the vaccine. If the rash were caused by the vaccine, the risk of transmission is very small; however, the child should avoid close contact with people who do not have evidence of varicella immunity and who are at high risk of complications of varicella, such as immunocompromised people, until the rash has resolved.
You cannot distinguish a mild case of varicella disease from a rash caused by the vaccine. The child may have been infected with varicella at about the same time s/he was vaccinated. The conservative approach would be to treat the child as if s/he had chickenpox and restrict her/his activities until all the lesions crust.
Breakthrough varicella represents replication of wild varicella virus in a vaccinated person. Although most breakthrough disease is very mild, the child is contagious and activities should be restricted to the same extent as an unvaccinated person with varicella disease.
Available data suggest that healthy children are unlikely to transmit vaccine virus. Transmission of vaccine virus to a household contact has rarely been documented. It appears that transmission of vaccine virus occurs mostly, or perhaps even exclusively, when the vaccinated person develops a rash following vaccination.
Transmission of varicella vaccine virus is rare. However, if a pregnant or immunosuppressed household contact of a vaccinated person is known to be susceptible to varicella, and if the vaccinee develops a rash 7 to 21 days following vaccination, it is prudent that they avoid prolonged close contact with the susceptible person until the rash resolves.
An 8-month-old is likely to have residual passive varicella antibody from his or her mother. The vaccine probably will have no effect, and no action is necessary. The dose should not be counted, and the child should be revaccinated on schedule at 12 through 15 months of age.