NEW YORK: Americans who need a yellow fever shot for travel may soon have a harder time getting the vaccine. A manufacturing issue has created a shortage of the only vaccine licensed in the US. The Centers for Disease Control and Prevention said Friday that doses could run out as early as next month. Officials are bringing in another vaccine that's used in other countries but it will be available at far fewer clinics. The vaccine is recommended for travelers to certain areas in South America and Africa, and about 20 countries require proof of vaccination for entry. For most people, only one dose of vaccine is needed in their lifetime. The yellow fever virus is spread by the same mosquito that transmits other tropical diseases, including Zika.