While Los Angeles County has made significant inroads in inoculating residents against COVID-19 — with more than 2.5 million residents fully vaccinated —18% of the region’s first-dose recipients are overdue for their second dose, officials said Monday.

Of those, 57% are between the ages of 18 and 64, according to Barbara Ferrer, the county’s public health director.

While there could be a number of reasons for the discrepancy, she noted that some people may have “simply forgotten” to get their second shot.

“Some people may have been put off by side effects of the first dose, or feel that one dose is as effective as they need, and is giving them enough protection,” Ferrer said.

Regardless, she urged anyone who got their first vaccine dose to return for their second, because vaccinations are contributing to declines in coronavirus hospitalizations and deaths in the region.

“Getting everyone fully vaccinated — which means two doses of the Pfizer or Moderna vaccine received, or one dose of the Johnson and Johnson vaccine — is our best hope for seeing these positive trends spread even more widely across all our communities,” Ferrer said. “Flu like symptoms that many people have for a very short while after vaccination are signs that your immune system is working. And while one dose does offer some protection, it’s not as strong a level of protection as what two doses offer.”

Ferrer highlighted that over 60% of seniors are fully vaccinated, while 23% of those 18 to 64 are inoculated.

And while vaccination rates are higher among seniors because they qualified to get the shots earlier, older residents getting the vaccine has led to a significant decline in hospitalizations and deaths among that age group, Ferrer noted.

Once scarce and hard to come by, vaccine appointments are open for residents 16 and older, with walk-ups available countywide this week.

“Let’s together spread better health throughout the county,” Ferrer said.