HONOLULU (AP) — The so-called COVID-19 delta variant, which was first detected in India, is spreading in Hawaii, state health officials said Tuesday.

Two cases on Oahu and one on the Big Island were associated with travel from the U.S. mainland. One case involved an Oahu resident with no travel history.

The state Department of Health said it is “investigating to determine the extent of household and community transmission."

The delta variant now represents more than 20% of coronavirus infections in the U.S. in the last two weeks, or double what it was when the Centers for Disease Control last reported on the variant’s prevalence.

Dr. Anthony Fauci, the nation’s top infectious disease expert, warned that the U.S. could be following the United Kingdom’s course, where the variant has become the dominant strain due to rapid spread among youth.

Fauci said indications are that the COVID-19 vaccines remain effective against the variant.

All but one of the four Hawaii cases involved unvaccinated people, said Dr. Sarah Kemble, acting state epidemiologist.

Hawaii's vaccination rate is 57%. Kemble noted the pace of vaccinations has slowed in recent weeks.