It is currently unclear as to what, if any, data was stolen.
South Korea's National Intelligence Agency privately briefed lawmakers about the alleged attack, reported local news agency Yonhap.
The BBC has asked Pfizer for a comment but it has yet to respond .
North Korea has yet to report a single case of coronavirus.
However, the country is due to receive two million doses of the AstraZeneca-Oxford vaccine in the coming weeks.
North Korea closed its borders in January last year soon after the virus began to emerge in China.
In November, Microsoft said at least nine health organisations including Pfizer had been targeted by state-backed organisations in North Korea and Russia.
It said North Korean groups dubbed Zinc and Cerium, and a Russian group nicknamed Fancy Bear, were responsible.
The Kremlin has denied targeting other countries' vaccine research.
While many of the break-in attempts failed, Microsoft warned at the time that some had been successful.
Since the start of the pandemic, states have been deploying their hackers as part of the coronavirus crisis.
Some of this has been intelligence gathering and some of it the theft of intellectual property to gain advantage.
North Korea was always likely to be one of the most active in this field.
It is a closed society but one with advanced cyber-units, which have been willing to target other countries not only for secrets but even just for money.
Last year in the UK, the National Cyber Security Centre threw a protective arm around vaccine research.
This year, the focus has shifted more towards protecting the supply chain and rollout of a vaccine.
And there is no doubt that because of what is at stake, the pharmaceutical and health sectors are going to be on the cyber front lines for the foreseeable future.