Boris Johnson shook hands with the Brazilian health minister Marcelo Queiroga shortly before the south American official tested positive for Covid on Tuesday.
Queiroga was seen interacting with the UK prime minister hours before Johnson went to meet US president Joe Biden at the White House.
Queiroga was also staying at the same hotel as Biden, who is 78, in New York City for the United Nations General Assembly.
Johnson was hospitalised with Covid in April 2020 but made a full recovery, while Biden is believed not to have contracted the virus at all throughout the pandemic.
The spokesperson for the UN Secretary-General refused to comment when approached by CNN Brazil, only noting they had “a contact tracing protocol in place in case of cases arising”.
The Brazilian minister also went to the UN to watch the Brazilian President Jair Bolsanaro’s speech, but so far all the other members of the Brazilian delegation have tested negative for Covid.
Queiroga told CNN Brazil that he would quarantine in New York for 14 days and would not leave the US with the rest of his party.
Some members of his delegation have cancelled their participation in the UN’s General Assembly to reduce the risk of infection.
Queiroga has been vaccinated with the Chinese jab, CoronaVac, but has not shared when he received the jab.
Bolsanaro’s presence caused a stir on Tuesday because he is prominently anti-vax – and Johnson proudly promoted the AstraZeneca vaccine while they were in a joint meeting.
Bolsonaro declared on social media only last week that he would not get the vaccine because he had natural immunity from catching the virus.
There have been concerns that the UN’s General Assembly could cause a spike in infections, especially as the States is struggling to overcome the virus.
The White House even asked world leaders to join only virtually to avoid a “super spreader event” last month – but more than 100 heads of state and government ended up arriving in New York this week.