Around 5 million Europeans have already received the AstraZeneca jab.
There have been about 30 cases in Europe of "thromboembolic events" - or developing blood clots - after the vaccine was administered. There were also reports that a 50-year-old man had died in Italy after developing deep vein thrombosis (DVT).
The WHO is investigating the reports, as it does any safety questions, Ms Harris said.
But no causal relationship had been established between the shot and the health problems reported, she said.
On Friday, AstraZeneca said the recorded number of blood clots in vaccinated people was "significantly lower... than would be expected among the general population".
"An analysis of our safety data of more than 10 million records has shown no evidence of an increased risk of pulmonary embolism or deep vein thrombosis," a spokesperson said.
Bulgaria's decision to pause its rollout followed similar steps by Denmark, Iceland and Norway as well as Thailand. Italy and Austria have stopped using certain batches of the drug as a precautionary measure.
The European Medicines Agency, the EU's medicines regulator, said earlier there was no indication the jab was causing the blood clots.