By Ania Lichtarowicz
BBC health reporter in Bangkok
The editor of the leading medical journal in the United States has accused the US administration of being petty by not allowing many of its scientists to attend the 25th International Aids conference in Bangkok.
Dr Catherine DeAngelsis told the BBC that this attitude is stifling research into HIV and free speech.
More than 20,000 delegates are expected to attend the conference
Far fewer US government employed scientists are attending the conference than in previous years.
But a government spokesman said financial considerations were an important factor in deciding how many scientists were sent abroad.
Dr DeAngelsis hit out after one researcher, whose work is being published in the Journal of the American Medical Association (Jama), was told that he could not come to Bangkok to present his work.
Dr Marc Bulterys, from the Centers for Disease Control and Prevention (CDC) in Atlanta, led a study into the transmission of HIV between mother and baby.
His team found that it was possible to test for HIV and give a woman in labour antiretroviral therapy (ARV) within just an hour and a half.
This research is published in a special Aids edition of Jama, and the leading researchers, who had contributed to this edition, presented their work at a special meeting in Bangkok.
But when it came to Dr Bulterys' work, Dr DeAngelsis, apologised to the audience for his absence.
She told the BBC that when the journal approached Dr Bulterys about presenting his research in Bangkok he said he could not come, and would not give a reason why.
"We had heard the US was going to limit the number of scientists because they said: 'Well all this great expense we could use that money to buy treatment.'
"So we offered to pay for it - and then he said: 'I can't come 'cos I'm not allowed'.
"It's an incredible example of political pettiness, it is anti-intellectual, it is interfering with science and the scientific process."
Dr DeAngelsis says a limited number of individuals have been selected and approved to come to Bangkok by someone in the government.
Tommy Thompson was booed at the last Aids Conference
There have been suggestions from a number of scientists that this policy was introduced because Tommy Thompson, US Secretary of Health and Human Services (HHS), was not well received at the last International Aids Conference two years ago.
Dr DeAngelsis added: "On further investigation, when I talked to people I know in Washington, it's because Tommy Thompson was booed in Barcelona.
"For someone from the United States to object to freedom of speech, this is ridiculous.
But Kathy Harben, a spokeswoman for the CDC, told BBC News Online financial considerations were an important factor.
"The limit is a reflection of the commitment of the HHS and CDC to achieving a balance of what we recognise as the importance of sharing and receiving new information at this international conference and also being good stewards of US tax payer dollars."
She added scientists could share information in other ways, such as by teleconferencing or by e-mail.
Professor Joep Lange, the president of the International Aids society and co-chair of the UNAids conference in Bangkok, confirmed that only 50 US government delegates will be attending the meeting.
He told the BBC this was a real pity for the people who cannot come as the US is a major player in Aids.
He also believes that it is because of what happened at the last Aids conference.
"In Barcelona, Secretary Thompson was booed down by US activists for more than half-an-hour. He actually couldn't speak. It's not the right reaction the US Administration is taking, but that has certainly had a negative impact on the willingness of the US Government to be present."