There is no long term carrier for poliovirus (carriers are people who carry the virus but have no symptoms of the disease), and polioviruses have no non-primate reservoir in nature. The virus cannot survive in the open environment for a long time. Therefore, interruption of person to person transmission of the virus by vaccination is the key step in removing polio entirely. The two vaccines have eliminated polio from most countries in the world. Polio worldwide has dropped from about 350,000 cases in 1988 to 1,652 cases in 2007.