With most seniors in Los Angeles County fully vaccinated against COVID-19, a greater proportion of younger adults are battling the disease in local hospitals, officials said Monday.

Over the past six weeks, there have been more 30- to 49-year-olds hospitalized with the virus than any other age group, compared to earlier in the pandemic when those who became sick enough for emergency care skewed older, Barbara Ferrer, L.A. County’s public health director, said in a briefing Monday.

“This is the first time over the course of the pandemic that we’ve seen this happen,” Ferrer said. “Before April, this age group consistently saw lower hospitalization rates when compared to other age groups.”

Ferrer said the trend represents a “gap in protection,” from vaccines and other factors.

Children and young adults have lower vaccination rates but are likely protected by virtue of their youth as well as by masking requirements at schools. However, many people in their 30s and 40s are workers and responsible for caring for others, which exposes them to risk, Ferrer said.

“If they’re not vaccinated, they are highly likely to end up with a COVID infection and unfortunately, as we see, disproportionately likely to end up in the hospitals,” she said.

More than half of L.A. County residents 16 and older have now received two doses of the vaccine, which Ferrer called a “thrilling landmark.” But she expressed concern that there continue to “large decreases” in the number of people getting a first dose.

“It’s clear that we need more folks coming in for the first dose appointments in order to create the conditions that can sustain our full reopening in a few weeks,” she said.

Countywide vaccination rates remain highest among those who have had access the longest, with 85% of those 65 and older vaccinated. Meanwhile the youngest teens have a vaccination rate of 15%, and 58% of all people ages 16 to 64 have had at least one dose.

There has been “significant progress” in vaccinating more people in some hard-hit areas, including South L.A. and East L.A., Ferrer said.

Most localities now have vaccination rates of at least 30%, and many are above 45%. But coverage in some areas still lags.

“In the west Antelope Valley, Palmdale and Pearblossom, for example, we’ve not yet achieved 30% vaccination levels,” Ferrer said.

Public health officials hope to vaccinate as many people as possible over the next few weeks as the county prepares to join California in fully reopening June 15.

Case, hospitalization and death rates remain on a downward trend. Four additional deaths and 139 new infections were reported Monday, but Ferrer said those figures reflect both the usual lag in weekend reporting and a delay in getting information from several local labs.