- Design
- Multicountry observational laboratory study
- Population
- 191 vaccinated, previously infected or unexposed adults
- Primary outcome
- Neutralising and binding antibody to mpox clades Ia, Ib and IIb
- Effect
- Neutralisation in 3–15% after MVA-BN alone; low clade Ib neutralisation after IIb infection
An observational laboratory study in The Lancet Infectious Diseases tested 232 plasma samples from 191 people in the USA, Brazil and Portugal: childhood smallpox vaccine recipients, MVA-BN recipients, people with both, people who had clade IIb mpox in 2022, and unexposed controls.
Antibodies from vaccinated people bound all three mpox clades, but neutralised them weakly. Detectable neutralisation was found in 3–15% of samples after MVA-BN alone, 28–35% after childhood smallpox vaccine and 39–55% after both. People who had clade IIb infection had broad binding but little neutralisation of clade Ib.
Neutralising antibody is only one part of protection, and MVA-BN was partly effective in the 2022 outbreak despite similar laboratory findings. These data do not show the vaccine fails against clade Ib. They do suggest that prior infection or vaccination should not be assumed to protect fully.
For clinicians, keep a low threshold for testing people with compatible lesions even if vaccinated or previously infected.
- Test for mpox in a compatible rash even if the patient was vaccinated or had mpox before
- Continue to offer MVA-BN where recommended; laboratory neutralisation is not the whole of protection
- Ask about travel to central Africa, where clade Ib has spread
Why it matters
It weakens the assumption that the 2022 response left people immune to the newer clade.
Don't overread it
Low neutralising antibody in the laboratory does not prove the vaccine fails to protect against disease.
The statistics, in plain English
PRNT50 measures how far plasma can be diluted and still halve viral plaques; values near the assay floor, as here, mean neutralisation was barely detectable.
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