There were no plans to vaccinate the general public, the Blue House spokesman said.

Adding to these concerns was a report in the Korean media alleging one of the four North Korean soldiers who defected in 2017 was found to have antibodies to the biological weapon in his system, suggesting he was either vaccinated against anthrax or exposed to it at some point.

South Korea's National Intelligence Service told CNN it could not confirm that report. The Defense Ministry said it also could not confirm the report, adding that none of the four military defectors are believed to have worked in North Korea's biochemical warfare unit.

Biological threat

In a statement, North Korea's Ministry of Foreign Affairs said such claims were "groundless" and accused the US of attempting to "cook up untruths."

"North Korea has the scientists and facilities for producing biological products and microorganisms, and has the ability to produce traditional infectious biological warfare agents or toxins," the report said.

Nerve agents and toxins

He was killed with the nerve agent VX, allegedly by two women acting on behalf of North Korea. Pyongyang has consistently denied any involvement in Kim's death; lawyers for the women have said they thought they were taking part in a prank TV show.

In 2015, North Korean state media published photos of leader Kim Jong Un touring a pesticide facility which experts argued was capable of producing large amounts of biological weapons.

"The North Korean assertion that the plant is intended to produce insecticides is an old and well-used cover for a biological weapons program," Hanham said, adding the publication of the photos of Kim's tour "may have been intended as a veiled threat to South Korea and the United States."

The Harvard report said "decades of open source information affirm that North Korea has held an interest in developing a bioweapons program," and there was evidence Pyongyang possessed the capability to produce, and in some instances weaponize, a number of biological agents, including anthrax, botulism, cholera, plague, and smallpox.

Devastating attack

Much about North Korea's biological weapons program is unknown, including whether it is still ongoing: Pyongyang has not been shy in boasting about its weapons capabilities, and it's difficult to judge that being a signatory to the BWC would be enough to encourage it to hide one controversial program while being increasingly open about its nuclear arsenal.

"Missiles, drones, airplanes, sprayers, and human vectors are potential means of (biological weapons) delivery," the Harvard report said, though it added biological agents are "difficult to keep intact on missile payloads due to heat and changing conditions that can degrade the agent," making far-range delivery unlikely.

Manual delivery is perhaps the most difficult to guard against, the report said: "North Korea has 200,000 special forces; even a handful of those special forces armed with (biological weapons) would be enough to devastate South Korea."