New York City’s monkeypox vaccine appointment website crashed on Tuesday “due to overwhelming traffic,” the city announced as the nation sees an uptick in cases.

“All available monkeypox vaccine appointment slots have been booked. Due to overwhelming traffic, as soon as appointments went online this afternoon, the site delivered error messages for many people who were unable to make appointments,” the New York City Department of Health and Mental Hygiene tweeted on Tuesday afternoon.

The city said that it would alert residents when more appointments could be scheduled and noted that the technicality difficulties experienced on their website “is just further proof that demand is very high.”

“We apologize for the frustration caused and are working to build stable appointment infrastructure as we roll out more appointments as vaccine supply increases in the coming weeks. We look forward to receiving more doses in the near future to provide to New Yorkers,” the city’s Department of Health and Mental Hygiene said in a separate tweet.

On the city department’s website page on monkeypox, it notes that there are no appointments available for those who want a vaccine, adding they expect later this week that more slots to signup should be available.

“Vaccine supply remains low. NYC is receiving a limited number of doses from the federal government, and we are making them available to New Yorkers as quickly as possible,” it reads under the “vaccination” section. 

Monkeypox, though rarely fatal, has milder symptoms compared to smallpox, according to the Centers for Disease Control and Prevention (CDC). It can be transmitted during sex or while kissing or when there’s direct contact with body fluids, scabs or rash, among other ways. 

CDC data as of Tuesday afternoon shows that there are 929 monkeypox cases nationwide.

New York City and Washington, D.C. have been offering the two-dose Jynneos smallpox vaccine to bisexual and gay men, gender non-confirming people and transgender women, among others. But both cities saw their vaccine supplies quickly max out the same day they started offering appointments.