CHICAGO (CBS) — The United Center mass-vaccination site will be switching later this month to single-shot Johnson & Johnson COVID-19 vaccines for all walk-in appointments.

The change begins on Tuesday, April 20.

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The Chicago Department of Public Health reported that it has had high demand from residents for the single-shot vaccine.

Appointments are required and will be available beginning Monday via Zocdoc, or through the city’s COVID Help Line at (312) 746-4835. The city emphasized that the appointments are likely to be in high demand and urged people to check back frequently.

The United Center drive-through site will still administer second doses of the Pfizer mRNA vaccine for people who already got their first dose of that vaccine there.

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Chicago will move to Phase 2 eligibility on Monday April 19 – meaning everyone 16 and over will be eligible to receive a vaccine. But only those 18 and older may receive the vaccine at a city-run site like the United Center.

Starting Monday, appointments for people eligible in Phase 2 will be posted through Zocdoc. New appointments will be added throughout the week.

Chicago is starting Phase 2 later than the rest of the state, for which it start this Monday.

The city emphasized that vaccinations at city-run sites through Zocdoc are limited to Chicago city residents. The vaccine is free to all, regardless of insurance or immigration status.

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The latest on the Chicago COVID-19 vaccine rollout is available at chicago.gov/covidvax.

CBS 2 Chicago Staff