Brazil's health ministry said its website was hit on Friday by a hacker attack that took several systems down, including one with information about the national immunisation program and another used to issue digital vaccination certificates.
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The government put off for a week implementing new health requirements for travellers arriving in Brazil due to the attack.
"The health ministry reports that in the early hours of Friday it suffered an incident that temporarily compromised some of its systems ... which are currently unavailable," it said in a statement.
Police said they are investigating the attack.
The alleged hackers posted a message on the website saying that internal data had been copied and deleted. "Contact us if you want the data back," it said, including e-mail and Telegram contact info.
The message had been removed by Friday afternoon, but the web page was still down, while user data in the ConectSUS app had disappeared.
The ministry said it was working to restore its systems.
Health Minister Marcelo Queiroga told reporters that no data will be lost, saying that the ministry holds all the information that was taken down.
Under measures decide on Tuesday after President Jair Bolsonaro opposed the use of a vaccine passport, unvaccinated travellers arriving in Brazil will have to quarantine for five days and be tested for COVID-19.
The requirement was due to start on Saturday but the government said that will be postponed for a week as vaccination data was not accessible on-line following the attack.
COVID-19 tracing forms for arriving airline passengers were still available on health regulator Anvisa's website that was not targeted.