NEW ORLEANS — A single dose of killed oral cholera vaccine provided short-term protection from newly endemic cholera, while the complete two-dose vaccination conferred long-term protection, a case-control study from the Haiti epidemic showed.

Getting a single dose in the two-dose vaccination campaign, which followed the 2010 outbreak that introduced cholera into Haiti, predicted vaccine effectiveness of 79% (95% CI 43-93) at 12 months, but then dropped to zero by 24 months, reported Louise Ivers, MD, MPH, of Harvard Medical School in Boston, and colleagues.

The average cumulative 4-year effectiveness for two doses was 76% (95% CI 59-86) in adjusted analyses presented here at the American Society of Tropical Medicine and Hygiene (ASTMH) meeting and simultaneously published in the Lancet Global Health.

“These results add to the evidence in support of the use of killed, bivalent, whole-cell oral cholera vaccines as part of comprehensive cholera control plans and add evidence to the investment case for continued support of a global stockpile of killed, bivalent, whole-cell oral cholera vaccines,” the researchers wrote.

Previous efficacy studies had suggested waning protection 2 years after oral vaccination, and no long-term prospective effectiveness data has been available from outside of Asia, “where cholera has been endemic for centuries,” the researchers noted.

“Cholera vaccination with the standard two-dose regimen can be challenging, especially in settings of crisis, conflict, or humanitarian emergency. Because of this, the use of a single dose has been proposed as a temporizing measure to reduce cholera risk in the short term, leading to an interest in understanding the protective effect of a single dose of oral cholera vaccines,” Ivers’ group wrote.

As an extension of an initial 2-year study, Ivers and colleagues assessed the effectiveness of the two-dose vaccination campaigns in Haiti using Shantha Biotechnics’ Shanchol killed, bivalent, whole-cell oral cholera vaccine using self-reported information. The researchers recruited patients from the area that were at least 12 months old, not pregnant, and living in the region during the vaccine campaign.

The study included 178 cases of cholera with positive stool culture among patients recruited from cholera treatment centers and 706 community controls matched by age group, time, and neighborhood.

Among them, 14% of cases and 11% of controls got only a single dose of vaccine.

There was no evidence of two-dose effectiveness declining during follow-up. For people age 5 and older, the adjusted, complete case analyses showed 4-year effectiveness of two doses of 77% (95% CI 58-88).

For a single dose, the adjusted effectiveness estimates declined log-linearly every month after the initial vaccine (P_interaction=0.0004). In the first year following vaccination, the observed high vaccine effectiveness varied from 96% to 79%. These data eventually decreased to zero after month 20.

The investigators acknowledged the limitations of their work: there was no set standard for assessing vaccine dose, there were a limited number of patients with a vaccination card to confirm their self-reported vaccination status, and only 12% of participants were under the age of 5 years.