Members of the international HIV research community were in a state of shock after the death of world-renowned scientist Joep Lange and at least several other activists who perished when Malaysia Airlines Flight 17 was shot down Thursday over eastern Ukraine.

In an address to the nation Friday at noon, President Obama said “nearly 100” of those killed — one third of the 298 people on board — may have been en route to the 2014 International Aids Conference in Melbourne, which is scheduled to begin this weekend. The White House said the figure came from remarks made by Australian Prime Minister Tony Abbott.

But conference organizers said they have only been able to confirm seven names.

“We have been working hard to try and confirm how many people were on the flight. We’ve been speaking to a number of different authorities, and we think the actual number is much smaller,” said Chris Beyrer, who will take over the presidency of the International AIDS Society at the end of the global conference next week.

Their efforts have been complicated by the fact that the airline has not released a passenger manifest so they have been relying on information from colleagues, friends, family members and authorities from various countries. He said that while there may be some people who were on the flight and headed to the conference that they do not know about, it appears that the numbers are “an order of magnitude smaller than what has been reported.”

Among the others who have been confirmed by employers or others to have been on the flight include three Dutch AIDS activists. Lucie van Mens had been involved in HIV/AIDS prevention work; Martine de Schutter was a program manager at Bridging the Gaps, which lobbies for universal access to HIV prevention; and Pim de Kuijer was a lobbyist at the group Stop AIDS Now.

The World Health Organization also confirmed that longtime spokesman Glenn Thomas had been on the flight on his way to the AIDS conference. In an e-mail, Oyuntungalag Namjilsuren, a colleague of Thomas’s at WHO, called the tragedy “a shell shocking event in global aids response.”

A member of the Dutch parliament was also killed on his way to the to the event, authorities said.

Lange’s partner, Jacqueline van Tongeren, who had worked in the field of AIDS for nearly 18 years, also died in the crash.

Beyrer called the loss of life “tragic and profound.” He called Lange, a preeminent Dutch HIV researcher and former president of the International AIDS Society, a “huge figure in the field who will be enormously missed.”

“I was just with him in Amsterdam a few weeks ago,” he said. Beyrer said Lange was a visionary who played in­cred­ibly important roles as one of the architects of combination retroviral therapy for HIV patients — a breakthrough that has made the virus something that is more akin to a chronic illness than a death sentence for many patients — and as an advocate for universal access to AIDS medicines.

Richard Elion, an HIV specialist and clinical research director at Whitman-Walker Clinic in Washington, described him as a great physician who “merged science with a social purpose and a sense of justice.”

“He is not a person who can be replaced, rather remembered with bittersweetness,” Elion said via his iPhone on his way to Melbourne.

The identities of the victims have not been confirmed by the airline.

About 14,000 delegates are expected at the conference.

AIDS research has been marked by previous aviation disasters. Irving S. Sigal, a molecular biologist who helped develop the drugs used to treat HIV, died in the explosion of Pan Am Flight 103 over Lockerbie, Scotland, in 1988. Ten years later, prominent researchers Jonathan Mann and Mary Lou Clements-Mann died in the crash of Swissair Flight 111 off the coast of Nova Scotia, Canada.

Staff writer Joel Achenbach contributed to this report.