New Delhi, Sep 21 (IANS): Amid the rising global outbreak of Mpox, two separate studies have shown that the efficacy of Bavarian Nordic’s vaccine against the deadly infectious disease wanes “to undetectable levels” in 6-12 months.

Modified Vaccinia Ankara-Bavarian Nordic or MVA-BN is indicated for active immunisation against smallpox, Mpox, and related orthopoxvirus infections and disease in all adults 18 years of age and older.

The vaccine, administered as a 2-dose injection given 4 weeks apart, is the first jab against Mpox to be prequalified by the World Health Organization (WHO).

The first study, published in the open-access medical journal Eurosurveillance, demonstrates that “orthopoxvirus-specific binding and MVA-neutralising antibodies waned to undetectable levels one-year post-vaccination in at-risk individuals who received two doses of MVA-BN”.

Led by researchers from the Erasmus University Medical Center, in the Netherlands, the study called for continuous surveillance to understand the impact of declining antibody levels.

The team investigated 99 high-risk active gay, bisexual, or other men who have sex with men (GBMSM) and 19 laboratory workers in the Netherlands in 2022 and 2023.

The participants included both those born before and after 1974 when smallpox vaccine ceased to be administered in the Netherlands.

“While antibodies induced by historic smallpox vaccination using first- or second-generation vaccines can be detected for decades, we find rapid waning of antibody in vaccine recipients without pre-existing immunity,” the study showed.

The other study, published on a preprint server and not peer-reviewed, also showed that “the MVA-BN vaccine generated mpox serum antibody responses that largely waned after 6-12 months”.

The study led by researchers from the Beth Israel Deaconess Medical Center at Harvard Medical School, US, collected serum samples from 45 participants who had either confirmed Mpox infection or received either one or two doses of MVA-BN.

In the study, Dan Barouch, the director of the Center for Virology and Vaccine Research at the Centre reported that vaccine-generated Mpox antibodies largely waned after 6 to 12 months.

After a year, antibody levels in participants with two doses of the Mpox vaccine were comparable or lower than peak antibody responses in people who received just one shot.

MVA-BN, developed in 2019, is a live attenuated vaccinia virus vaccine. A recent study in the British Medical Journal showed that the vaccine efficacy estimates for two doses of the vaccine range from 36 per cent to 86 per cent.

Having just one dose is only about 58 per cent effective in preventing Mpox, the study showed.