The San Diego Immunization Survey (SDIS) is conducted annually by the County of San Diego Health and Human Services Agency's Immunization Branch (SDIB). Since 1995, the SDIS has provided annual vaccination coverage estimates for 19- to 35-month-old children living in San Diego County, California. The SDIS uses a combination of parental input, registry data, and provider verification to estimate coverage in San Diego. San Diego County uses annual estimates of vaccination coverage obtained from the SDIS to monitor and improve immunization services in its community.

This survey could be used by other localities to assess coverage to improve immunization services and vaccination rates. The resources a locality would need to dedicate to performing such a survey may be validated by the benefit it can have toward decreasing the risk of vaccine-preventable diseases in the community.

In the first phase, a random-digit-dialing (RDD) telephone survey is conducted annually between March and June. The RDD methodology is used to identify households in San Diego that have a 19- to 35-month-old child. When a household with an appropriately aged child is identified, demographic information about the household is obtained from the RDD respondent. Eighty-five percent (711/839) of the 2005 SDIS respondents were the mothers of the age-eligible children identified in the household. If the respondent has a state-issued written immunization record for the child, he/she is asked to read from that record the dates when the child received doses of diphtheria, tetanus toxoids, and acellular pertussis (DTaP), polio, measles-mumps-rubella (MMR), Haemophilus influenzae type b (Hib), hepatitis B (HepB), varicella (VAR), pneumococcal conjucate vaccine (PCV), hepatitis A (HepA), and influenza (flu) vaccine. The respondent is also asked to read the name and contact information for the provider that administered each dose. If the written immunization record used during the RDD interview indicates that the child was fully vaccinated, the SDIB considers the report to be a sufficient documentation of immunization history and further information is not requested from either the child's vaccination provider or the registry.

If an immunization record is not available, then the number of doses is recorded from the respondent's memory. In the 2005 SDIS, a child was considered to be fully vaccinated if information from the immunization record showed that the child had received doses approximately at the recommended ages and intervals of ≥4 doses of DTaP, ≥3 doses of polio, ≥1 dose of MMR, ≥3 doses of Hib, ≥3 doses of HepB, ≥1 dose of VAR (or history of varicella disease), ≥3 doses of PCV, 1 dose of HepA if the child was ≥24 months of age or 2 doses of HepA if the child was ≥30 months of age and more than six months had passed since his/her first dose, and ≥1 dose of flu vaccine. If the immunization record did not indicate that one dose of flu vaccine was administered, then the respondent's recall was accepted. Surveys in years prior to 2004 do not include all of these vaccines. As new vaccines become available and recommended, the SDIB adds them to the list of vaccines that it monitors through the SDIS.

The second phase of data collection is conducted if the RDD respondent's report of the child's vaccination history is not read from an immunization record or if data from the immunization record do not show that the child is fully vaccinated. Consent to contact the child's provider is requested verbally from all respondents during the telephone interview. If consent is obtained and second-phase data collection is warranted, the San Diego Regional Immunization Registry (SDIR) is queried to verify the child's vaccination history. If a complete immunization record cannot be acquired using SDIR, then it is requested from the child's medical provider(s). The SDIB attempts to contact local, out-of-state, and out-of-country providers for verification. Although providers typically comply with the request for verification, some ask for documentation. In these situations, SDIB sends a form with the interviewer's signature on it stating that verbal consent was received from the respondent during the telephone interview. If the provider insists on receiving written consent from the respondent, SDIB is not able to obtain the information necessary to verify immunization status.

Estimates of vaccination coverage from the SDIS indicate that the estimated percentage of children who received at least the recommended number of doses for DTaP, MMR, polio, Hib, and HepB increased from 1995 to 2005 (Table 1). In 2005, coverage estimates for the 4:3:1 series (≥4 doses DTaP, ≥3 doses polio, ≥1 dose MMR), 4:3:1:3:3 series (≥4 doses DTaP, ≥3 doses polio, 1≥ dose MMR, ≥3 doses Hib, ≥3 doses HepB), and 4:3:1:3:3:1 series (≥4 doses DTaP, ≥3 doses polio, ≥1 dose MMR, ≥3 doses Hib, ≥3 doses HepB, ≥1 dose VAR) were 87.8%, 84.9%, and 82.8%, respectively. Children ≥24 months of age had higher coverage rates than children <24 month of age. From 2002 to 2005, coverage for ≥1 dose of VAR exceeded 90% annually. Additionally, a substantial increase from 68% to 86% for ≥3 doses of PCV from 2004 to 2005 was observed.

Because the SDIS accepts a complete, valid immunization history read by the respondent from an immunization record as proof of immunization status, it relies less on obtaining vaccination histories from providers than the NIS. As a result, the SDIS is less resource intensive compared with the NIS.

Because the NIS is a survey that collects data monthly, it collects data on more birth cohorts than the survey conducted by San Diego, which collects data only from March through June. Therefore, the coverage estimates from these surveys are only approximately comparable with respect to the cohorts included in their samples. In 2000, 2002, 2003, and 2004, the estimated 4:3:1:3:3 coverage rates for the San Diego survey were significantly higher by 11.6%, 9.2%, 6.4%, and 5.7%, respectively, than national estimates produced by the NIS, while there was no significant difference in 2004.¹⁰