"I think with any immunization campaign, conflict in an area really increases the challenge of reaching children," said Dr. Julie L. Gerberding, director of Centers for Disease Control and Prevention.
Still, the directors of the Measles Initiative say eradication of measles may become more feasible as more and more milestone goals are met.
"We do ask ourselves the same question, whether it is realistic now to talk about eradication given the complexity of factors affecting the epidemiology of the disease and the extremely high level of immunization coverage that has to be reached," Chan said.
"At this stage I think it is too early to talk about eradications, but it is something that we can talk about and see the next stage after we have achieved the new goal."
Regardless of whether eradication is possible, there is little doubt that the Measles Initiative has made a dramatic impact on the world measles picture.
More than half a million fewer deaths from measles were seen in 2005 than in 1999. And cumulatively, Chan said, immunization efforts through the Initiative have saved 2.3 million lives so far.
But now that measles is on the ropes, the question is whether this effort can be sustained into the future to deliver a knockout blow.
"At the moment I think we can celebrate the fact that we've been wonderfully effective in decreasing death from the disease," Katz said.
"But to maintain progress of this sort it requires an incredible commitment -- one that has to be sustained every year. You can't just do it and walk away, because if there is one person around who still has the virus, it can be spread."
Imperato said that sustaining the progress would also required a continued effort to not only immunize against measles, but also to address the other factors that contribute to measles mortality in developing nations.
"When we have an outbreak over here, we have a mortality rate of less than 1 percent," he said. "Why do they have a 10 percent or higher mortality rate in Sub-Saharan Africa? The difference is not that the virus in any different; the difference is the children. In these children it's not just measles; measles is what tips the balance."
Other factors, including malnutrition, parasitic infections and other diseases such as cholera, make measles much more deadly. And lack of adequate infrastructure and amenities worsens the problem.
"Measles was the only childhood disease I remember having because I was so sick," Imperato said. "So you can imagine a poor child in a developing country, living in a place with no running water or electricity, running a temperature of 110 degrees and malnourished."
The Initiative will also need continued funding. The effort so far has cost an estimated $308 million, said Bonnie McElveen-Hunter, chairwoman of the American Red Cross, at Thursday's press conference.
And the 90 percent reduction targeted for 2010 will take about $500 million more. Of this total, Chan said the initiative has currently secured $140 million to $150 million.
Still, directors of the effort said they are confident that the Measles Initiative will continue to drive down measles deaths.
"I'm very hopeful that we will be successful, and I think we have every reason to be hopeful," said Gerberding.