Pakistan Prime Minister Imran Khan tested positive for Covid-19, his national health adviser said on Saturday, two days after Mr Khan was vaccinated.
Mr Khan, 68, is "self-isolating at home", Faisal Sultan said in a tweet.
The prime minister's office confirmed the diagnosis.
"At this point, the prime minister's office can only confirm that the honourable prime minister has tested positive for Covid-19 and has self-isolated. We will release more details in due course," it said.
Indian PM Narendra Modi has sent his 'best wishes', following tentative moves between the two governments towards rapprochement.
Mr Khan's office shared a video on Twitter of him being vaccinated on Thursday, along with a message from the prime minister urging people to follow health protocols as the country faced a third wave of coronavirus infections.
Pakistan's Health Ministry said Mr Khan had not had time to develop antibodies after being vaccinated.
"He only got the first dose merely two days ago, which is too soon for any vaccine to become effective," the ministry said on Twitter.
Mr Khan had a number of public engagements in recent days, including the inauguration of a security conference in Islamabad on Wednesday when he spoke without wearing a mask.
On Friday he attended a ceremony to inaugurate a housing project for the poor.
Mr Khan also received Kuwaiti Foreign Minister Sheikh Ahmed Al Sabah on Thursday, his office said.
Pakistan is reporting a sharp increase in coronavirus infections. On Saturday the government reported 3,876 new cases in the previous 24 hours – the highest number of daily infections since early July.
There were also 42 more deaths, taking the total to 13,799 from more than 620,000 Covid-19 cases so far.
Pakistan, with a population of 220 million, is largely reliant on the Covax vaccine sharing initiative for shots.
The country is using more than one million doses of Sinopharm donated by China to vaccinate frontline health workers and the older population.
Narendra Modi sends good wishes
In a move that surprised some oberservers on Saturday, Indian PM Narendra Modi said he wished Mr Khan a "speedy recovery".
Since agreeing to halt hostilities on their disputed Kashmir border last month, the nuclear-armed rivals have shown signs of wanting to improve relations.
Mr Modi used the announcement that the 68-year-old Mr Khan had tested positive for the coronavirus to make his gesture.
"Best wishes to Prime Minister Imran Khan for a speedy recovery from COVID-19," the Hindu nationalist leader said on Twitter.
Mr Modi regularly uses his rallies to lash out at Pakistan.
The two countries — which fought intense artillery battles on their Kashmir border last year — have had three wars since their split after independence in 1947.
But the surprise accord reaffirming a 2003 ceasefire came amid reports that officials from the two sides had held secret meetings in recent months.
There have been no reported shellings on the Kashmir Line of Control, as the disputed frontier is known, since the accord came into effect on February 24.
Mr Khan has since called for increased trade with India, while Pakistan's army chief General Qamar Javed said this week that the neighbours should "bury the past".