Pakistan began to vaccinate 5.5 million children against pneumococcal disease to fight pneumonia on Tuesday. Pneumonia kills tens of thousands of youngsters in the country every year.
Pakistan is the first country in South Asia to launch a campaign against the disease, according to UN children's agency UNICEF, which is working with the government on the project.
More than 325,000 under-fives die in Pakistan every year, UNICEF said, and the lung condition is among the main culprits.
"Pneumonia is one of the leading killers of Pakistani children, accounting for about 19 percent of child mortality," UNICEF spokesman Zeeshan Suhail told AFP.
Zahid Larek, the head of Pakistan's expanded immunisation programme, told AFP pneumonia claimed the lives of more than 35,000 children every year.
Vaccination will begin this week in central Punjab province, which is home to around 60 percent of the country's population of around 180 million.
The southern province of Sindh and Pakistan-administered Kashmir will follow later in the year.
Confidence in vaccination programmes in Pakistan was damaged when the CIA used a hepatitis campaign as cover for an effort to gather DNA samples in its hunt for Osama bin Laden, killed in the garrison town of Abbottabad in May 2011.
Prime Minister Raja Pervez Ashraf has given his backing to the pneumococcal campaign to try to convince people to accept the vaccine.
Bin Laden's killing resulted in a boycott of polio vaccination in the Taliban and Al-Qaeda infested tribal region bordering Afghanistan.