Vaccination of 2-Year-Olds in the U.S.-Affiliated Pacific Islands, 2016

Vaccine-preventable diseases cause substantial illness and death in the U.S.-Affiliated Pacific Islands (USAPI). Their remoteness — especially of the outer islands — makes delivering vaccines hard and strains limited resources, and immunization staff also cite widespread socioeconomic disadvantage, hard-to-reach clinics and highly mobile populations. The United States keeps a military presence in the region, and USAPI citizens can live, work and travel in the United States without restriction, so outbreaks in the islands have been linked to imported cases and outbreaks on the mainland. The islands receive economic aid, immunization infrastructure support and some vaccines through Section 317 of the Public Health Service Act, plus CDC technical help.

A new way to measure coverage

In 2016, CDC and the island immunization programs piloted estimating coverage by abstracting medical records — the first simultaneous assessment of childhood vaccination across five jurisdictions: American Samoa; Chuuk State, Federated States of Micronesia; the Commonwealth of the Northern Mariana Islands (CNMI); the Republic of the Marshall Islands (RMI); and the Republic of Palau.

Records for children aged 24–35 months were gathered from delivery logbooks, birth rosters, medical records, public health vaccination logs and electronic systems, then matched and merged into one record per child. Children counted as up to date if they had received all doses recommended by 24 months under their jurisdiction's own schedule. The full series was at least 4 doses of DTaP, 3 of polio vaccine (IPV), 1 of MMR, 3 of Hib, 3 of hepatitis B and 4 of pneumococcal conjugate vaccine (PCV), written 4:3:1:3:3:4.

Vaccine-preventable disease outbreaks in the U.S.-Affiliated Pacific Islands, 2002–2018. CDC figure.

Results

• Hepatitis B birth dose coverage exceeded 85% everywhere except Chuuk (53.5%).
• Every other vaccine fell short of the jurisdictions' 90% targets, except in Palau, which reached 94.6% for IPV, 93.1% for Hib and 93.1% for hepatitis B.
• Coverage varied widely: 3 doses of IPV ranged from 94.6% (Palau) to 58.9% (CNMI), and 1 dose of MMR from 88.4% (Chuuk) to 57.9% (CNMI).
• Coverage dropped with each later dose. Three doses of DTaP ranged from 94.6% (Palau) to 59.5% (CNMI), but four doses from 79.9% (Palau) to 36.7% (Chuuk).

Main islands and outer islands

In Chuuk and RMI, children on outer islands were less protected for most vaccines.

• Chuuk: coverage was higher on the main island, Weno, for every vaccine except MMR, by 10.1 to 30.6 percentage points. Full-series coverage was 36.7% on Weno and 15.9% on the outer islands. The hepatitis B birth dose showed the widest gap: 81.2% on Weno against 45.4% on the outer islands.
• RMI: Ebeye, one of two main islands, had coverage 15.3–51.4 points higher than Majuro, the other, for all vaccines but the birth dose, and 25.3–66.8 points higher than the outer islands. Full-series coverage was 67.6% on Ebeye, 39.0% on Majuro and 18.5% on the outer islands. The hepatitis B birth dose reached 94.9% on Majuro and 88.9% on Ebeye, but 41.3% on the outer islands.

What it means

Coverage was below every jurisdiction's targets, and doses due in the second year of life, such as the fourth doses of DTaP and PCV, lagged well behind infant doses. So many under-vaccinated children let outbreaks spread fast and recur: there have been at least 13 documented outbreaks in the islands since 2000, and cases reach the mainland often.

Distance from health care likely explains much of the island gap. The hepatitis B birth dose is given at or soon after delivery in a clinical setting, but nearly 60% of outer-island children are born at home, against 11% on Ebeye and under 4% on Majuro, and about 70% of outer-island children appeared only in outreach logbooks, suggesting they might never have visited a health facility on Majuro or Ebeye. In Chuuk, MMR coverage was similar on the main island (86%) and outer islands (89%), perhaps thanks to a 2014 mass MMR campaign for everyone aged 6 months to 49 years during a measles outbreak.

Household surveys had been too costly and slow to run often, so this record-based assessment offers a baseline. Island programs were weighing better vaccine inventory management, eliminating missed vaccination opportunities, reminder and recall systems for second-year doses, and outreach to caregivers. Palau was considering more well-child visits, FSM was improving vaccine forecasting and running catch-up campaigns in all states, and RMI was adding immunization staff. Several islands designed, funded and launched interventions within a year of receiving the results.

Limitations: children missing from every record were left out; children who had died or moved away but were still on the books were counted (4% of people targeted in one FSM campaign had moved away); only 2-year-olds were assessed; and differing schedules make comparisons between jurisdictions hard.

Sources

• Tippins A, Murthy N, Meghani M, et al. "Vaccination Coverage Among Children Aged 2 Years — U.S. Affiliated Pacific Islands, April–October, 2016." MMWR 2018;67(20).