As California looks to ramp up efforts to immunize its millions of residents against COVID-19, health officials say they continue to run into a stubborn hurdle: the inability to secure a sufficient and reliable stream of vaccine doses.

The problem is twofold, officials say. First, there’s a limited supply to begin with, and some areas of the state, including Los Angeles County, have actually seen their shipments shrink over the last few weeks.

Second, both of the currently available vaccines require two doses taken weeks apart. So, the more initial doses that are doled out, the bigger the need will be for follow-up shots down the line — creating a chokepoint that constrains how many new people can begin the vaccine regimen.

Recently, some people seeking a first shot have even had their appointments canceled because vaccines were in short supply or needed for second doses.

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