In updating immunizations for immigration (“green card”) exams, I regularly come across intervals between catch-up vaccine doses that are shorter than ACIP recommendations—most often the last 2 doses of IPV are given less than 6 months apart, but also sometimes the 2 doses of varicella are given less than 3 months apart, and the next-to-last and last Td are given less than 6 months apart. How significant is this in terms of immunity?

The significance of non-standard intervals probably depends on the vaccine and the dose. This is a complex issue—studies have not been done to examine the effect of various intervals between doses on the immunogenicity of those doses. But ACIP has examined the available data and made recommendations about the minimum acceptable interval between doses for that dose to be considered valid (there is no maximum interval between doses). These minimum intervals are published as Table 3-2 in ACIP’s “General Best Practice Guidelines on Immunization”, available at www.cdc.gov/vaccines/hcp/acip-recs/general-recs/timing.html. Doses with a minimum interval less than the recommended minimum, as described in Table 3-2, should not be counted as valid. More details on this topic can be found in the General Best Practice Guidelines.