No one immediately claimed responsibility for the attack.
Three medics were wounded and taken to hospitals by police, said Ferdaws Faramarz, spokesman for the Kabul police chief.
The bus was taking the doctors, nurses and other health workers from Kabul toward northern Panjsher province when it was attacked in the northern district of Kalakan in Kabul province, he added.
No one immediately claimed responsibility for the attack, and Faramarz said police were investigating.
The attack comes a few days after the remaining 2,500 to 3,500 American troops officially began leaving the country. They will be out by Sept. 11 at the latest. The pullout comes amid a resurgent Taliban, who control or hold sway over half of Afghanistan.
The top US military officer said Sunday that Afghan government forces face an uncertain future and possibly some "bad possible outcomes” against Taliban insurgents as the withdrawal accelerates in the coming weeks.
A sticky bomb attached to an armored vehicle in northern Kabul killed two, and wounded at least two others, according to Ferdaws Faramarz, a spokesman for the Kabul police chief. No further details were immediately available.
Fawad Aman, deputy spokesman for the Afghan Defense Ministry also says three other soldiers were wounded in Thursday's attack in the Chahar Asyab district in Kabul province.
Daesh said one of its fighters blew himself up at a "large gathering" in Kabul while others "detonated a parked explosives-laden vehicle" when security forces arrived, in a statement posted on the Telegram messaging app.
Angry mobs in eastern India set fire to train carriages on Wednesday in protests over access to railway jobs that have seen police violently disperse crowds with tear gas and baton charges.
A summit was held in Abu Dhabi that brought together Sheikh Mohamed Bin Zayed, King Hamad and Abdel Fattah Al Sisi. The three leaders reviewed the terrorist attacks carried out by the Houthi militia on civil sites and facilities in the UAE.
The world's first medical trial authorised to deliberately expose participants to the coronavirus is seeking more volunteers as it steps up efforts to help develop better vaccines.