The US Centers for Disease Control and Prevention estimated that Ebola cases in these two West African nations could range from between 550,000 and 1.4 million cases by January 20, 2015.
The estimate is based on the assumption that Ebola cases in the world's largest outbreak to date are being underreported by a factor of 2.5, said the report from the CDC.
However, experts cautioned that the projection was based on data available in August, before the United States ramped up its response to the epidemic in West Africa, which has killed about half of the 6,000 people infected.
"Modeling suggests that extensive, immediate actions -- such as those already started -- can bring the epidemic to a tipping point to start a rapid decline in cases."
The UN health agency earlier today said that new cases would increase from hundreds each week to thousands without "drastic improvements in control measures," with infections more than tripling to 20,000 by November.