The vast majority of its 67,000 US staff have supplied proof of vaccination, which was required by Monday.
“This was an incredibly difficult decision,” its bosses said in a memo to employees.
The Chicago-based airline set out its Covid requirements for staff in August.
Its US employees had to upload proof of vaccination, or the first of two jabs, by the deadline on Monday.
The 593 workers who have refused a coronavirus vaccine and have not applied for an exemption on religious or medical grounds now face losing their jobs.
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“Our rationale for requiring the vaccine for all United’s US-based employees was simple – to keep our people safe – and the truth is this: everyone is safer when everyone is vaccinated, and vaccine requirements work,” its chief executive Scott Kirby and president Brett Hart said on Tuesday.
“This was an incredibly difficult decision but keeping our team safe has always been our first priority,” they said.
Some of those employees could be kept on if they have been jabbed and have simply failed to submit proof of vaccination – or if they are vaccinated before formal meetings on the matter, the company said.
United said it would follow the rules outlined in union agreements on the dismissals. The process could take weeks or months.
A further 2,000 employees have requested an exemption to the policy.
It previously said it would put those who are exempt on temporary, unpaid leave from 2 October. But those plans were put on hold after a lawsuit was filed by six employees challenging the policy.
Fiona Cincotta, the market analyst at City Index, told the BBC’s Today program that the “strict” policy was not likely to be introduced by UK airlines.
Like many companies in the aviation sector, United was severely hit by pandemic-related travel restrictions.
At the height of the crisis, it announced that it would need to furlough up to 36,000 staff.
It denied, however, that its vaccine policy would affect recruitment going forward, although vaccination will be a condition of hire for new staff.
On Tuesday it said that it had received more than 20,000 applications for about 2,000 flight attendant jobs.
Elsewhere in the US, few airlines have introduced vaccine mandates for their staff. Delta Airlines, for example, has announced a $200 (£148) monthly health insurance surcharge for those who are not jabbed.