M&G,
There have been studies done on Chick-V since its first diagnosed outbreak back in 1952 - over 60 years ago. This is from the World Health Organization:
"Chikungunya is a mosquito-borne viral disease first described during an outbreak in southern Tanzania in 1952. It is an RNA virus that belongs to the alphavirus genus of the family Togaviridae."
Long term effects are known. The reason the CDC says that an infected person is "most likely" immune is that over 60 years of studies have proved it to be true with this as well as other RNA viruses. They are saying "most likely" because they are essentially "covering their ass" so to speak. Just like mumps, chicken pox, etc, there have been rare exceptions to the rule and a few people have contracted these viruses more than once. A friend of mine growing up was written up in a medical journal because he caught chicken pox twice. But he was the exception to the rule - thus the interest in the medical journal.
Countries and territories where locally contracted chikungunya cases have been reported:
AFRICA
Benin
Burundi
Cameroon
Central African Republic
Comoros
Democratic Republic of the Congo
Equatorial Guinea
Gabon
Guinea
Kenya
Madagascar
Malawi
Mauritius
Mayotte
Nigeria
Republic of Congo
Reunion
Senegal
Seychelles
Sierra Leone
South Africa
Sudan
Tanzania
Uganda
Zimbabwe
EUROPE
Italy
France
OCEANIA/PACIFIC ISLANDS
American Samoa
Federal States of Micronesia
New Caledonia
Papua New Guinea
Tonga
ASIA
Bangladesh
Bhutan
Cambodia
China
India
Indonesia
Laos
Malaysia
Maldives
Myanmar (Burma)
Pakistan
Philippines
Saudi Arabia
Singapore
Sri Lanka
Taiwan
Thailand
Timor
Vietnam
Yemen
AMERICAS
Anguilla
Antigua and Barbuda
Aruba
Bahamas
Barbados
Belize
Bermuda
Brazil
British Virgin Islands
Cayman Islands
Colombia
Costa Rica
Curacao
Dominica
Dominican Republic
El Salvador
French Guiana
Grenada
Guadeloupe
Guatemala
Guyana
Haiti
Honduras
Jamaica
Martinique
Mexico
Montserrat
Nicaragua
Panama
Paraguay
Puerto Rico
Saint Barthelemy
Saint Kitts and Nevis
Saint Martin
Sint Maarten
Saint Lucia
Saint Vincent and the Grenadines
Suriname
Trinidad and Tobago
Turks and Caicos Islands
United States
US Virgin Islands
Venezuela