A Measles Scare That Wasn't: Rash Illness in American Samoa, 2023

On April 24, 2023, the American Samoa Department of Health declared a public health emergency over a possible measles outbreak. Two children with rashes had tested positive for measles immunoglobulin M (IgM) antibodies — and the results arrived just after a huge island-wide celebration. The investigation that followed, reported in CDC's MMWR, found no measles at all.

American Samoa, a remote U.S. territory in the Southern Hemisphere (MMWR).

Why officials were alarmed

• On March 23, 2023, an 8-year-old child with two doses of MMR vaccine and no fever was seen for a three-day itchy rash all over the body. Nearly 20 days later, a 4-year-old, also fully vaccinated and without fever, was seen for a rash. Both tested positive for measles IgM at a commercial lab in California.
• The results reached the health department after Flag Day on April 17, an island-wide event with large gatherings.
• In 2019, a large measles outbreak had swept several Pacific Island countries, including American Samoa; neighboring Samoa, where vaccination coverage was low, had nearly 6,000 cases and more than 80 deaths.
• In January 2023, only about 72% of 6-year-olds in American Samoa (population about 50,000) had received two doses of MMR.

The emergency declaration closed schools and child care centers, launched a territory-wide MMR vaccination campaign, imposed travel restrictions, started active case finding and brought in CDC technical help.

The investigation

From March to July 2023, health officials evaluated 86 people for measles, mostly children (median age 5.9; 95% younger than 13). More than half — 51 — didn't meet the standard clinical case definition for measles, but their rashes prompted testing anyway. Of the 35 who did meet it, 31 had received at least one MMR dose and 18 had received two.

Testing was hard. American Samoa had no validated measles tests, so specimens had to be flown off-island, with long travel times and few flights. No nose or throat swabs — the best specimens for detecting measles virus genetic material — were collected from the first two children, so they couldn't be retested. Later specimens went to the Hawaii State Laboratories Division and on to CDC.

What the tests showed:

• All PCR tests for measles were negative, except one from a recently vaccinated child — and CDC's MeVA test, which detects only vaccine strains, showed that result matched vaccination 11 days earlier.
• Four people had positive measles IgM and IgG results. In two, high IgG avidity and low neutralizing antibody levels pointed to false-positive IgM (real recent measles would produce high neutralizing antibodies). Another's results suggested recent vaccination, and the registry later confirmed an MMR dose a month before the rash.
• Of 37 nose and throat swabs tested at CDC, 32 were positive for some other virus — most often parvovirus B19 (19), which causes fifth disease, along with rhinovirus, human herpesvirus 6, influenza B, RSV and SARS-CoV-2. Many had more than one.

The emergency ended on June 8, 2023.

Why measles tests can mislead

In an unvaccinated person, measles typically brings 3–5 days of fever with cough, coryza or conjunctivitis, followed by a generalized rash that spreads downward. Similar rashes come from fifth disease (parvovirus B19), hand-foot-and-mouth disease (usually coxsackievirus), roseola (human herpesvirus 6), human herpesvirus 7, rubella and other viruses.

• Where measles is rare, a positive IgM test is less likely to mean real measles, raising the risk of false positives.
• Some IgM tests cross-react with other childhood viruses — parvovirus B19 in particular.
• Vaccinated people can get a rash, and standard tests can't tell a vaccine rash from measles; MeVA testing can, even during a vaccination campaign.
• IgG avidity testing helps separate recent from past immunity, and plaque reduction neutralization is the benchmark test for measles-neutralizing antibodies.

Lessons

• For every suspected measles case, collect both a nose or throat swab for molecular testing and blood for antibody testing.
• Reserve measles IgM testing for cases that meet the clinical case definition — especially people without evidence of immunity and with recent international travel.
• Where vaccination coverage is low, take infection-control measures while awaiting confirmation.
• Build local lab and surveillance capacity. American Samoa is doing so with the Lyndon B. Johnson Tropical Medical Center, its only hospital — new tests, training, a measles response manual, and better case-based and syndromic surveillance — a potential model for other Pacific Islands.
• Above all, high measles vaccination coverage prevents outbreaks.

Sources

• Stefanos R, Schatzman S, Wakeman B, et al. "Pediatric Rash Illness Outbreak with Initial Positive Measles Immunoglobulin M Antibody Test Results — American Samoa, March–July 2023," MMWR Vol. 73, No. 45, CDC; a work of the United States government in the public domain.