The delta variant of the coronavirus, which has quickly become the dominant strain in other countries and can cause more severe illness, is now spreading in Hawaii, prompting state heath officials Tuesday to once again urge Hawaii residents to get vaccinated not just to protect themselves, but the broader community, including young children who aren’t yet eligible for the vaccine.

Officials with the state Department of Health have identified only four cases of the highly contagious variant but told reporters during a news conference Tuesday that’s likely an undercount. The state’s genome sequencing, which screens for variants, provides a snapshot of what was occurring three weeks ago.

“We think that probably during the ensuing three weeks, it spread even further,” said State Laboratories Division Administrator Edward Desmond. “So this is a matter of concern.”

Desmond said it takes three weeks to identify specimens, collect them from labs and then complete the genome sequencing.

State health officials announced earlier this month that a case of the delta variant had been identified in a fully vaccinated Oahu resident who had traveled to Nevada in early May. While such “breakthrough” cases are rare, health officials said, they aren’t unexpected as the vaccines are not 100% effective.

The other three cases that have since been identified are all in people who were not vaccinated against the coronavirus, said acting state Epidemiologist Sarah Kemble.

Two of those cases involved travel to the mainland.

Kemble said that the Health Department is investigating the cases, and she didn’t have information about whether the unvaccinated travelers underwent testing before returning to Hawaii or complied with state quarantine requirements. One of the cases was not associated with travel, suggesting that the virus is spreading within the community.

The Health Department is investigating the cases to determine the extent of household and community transmission. Three of the cases are on Oahu, and one is on Hawaii island.

While none of the individuals required hospitalization, they did have symptoms.

Dr. Anthony Fauci, the nation’s top infectious disease expert, has called the delta variant the “greatest threat” to the country’s efforts to eradicate the coronavirus. While the variant made up just 10% of new cases in the U.S. two weeks ago, he said that figure has jumped to 20%.

Health officials have warned that the variant, which was first detected in India, could become the most dominant strain in the U.S. That’s a concern because of how easily it spreads, as well as its potential to make people sicker. A recent study from Scotland found that the risk of hospitalization was about double that of the B.1.1.7 strain, also referred to the alpha or U.K. variant. Doctors in China have also reported that as the delta variant spread, they were seeing patients becoming sicker and their conditions worsening more quickly than with other strains.

Kemble said with the spread of the delta variant, the state could see additional case surges and outbreaks in people who remain unvaccinated. Of particular concern are children under 12 who are not yet eligible for the vaccine.

Kemble stressed that the best way to protect young children is for their households and the broader community to get vaccinated to help halt the virus’ spread. The state has set a goal of vaccinating 70% of Hawaii’s population before dropping its COVID restrictions on activities and businesses. But the pace of vaccinations has been slowing. As of Tuesday 62% of the population had received at least one does of a vaccine, while 57% had been fully vaccinated.

“I do worry about our keiki in Hawaii,” said Kemble.