- Design
- Prospective cohort
- Population
- 544 pregnant women in outbreak areas of Rio de Janeiro state, Brazil
- Primary outcome
- Adverse maternal and infant outcomes
- Effect
- Maternal aRR 2.90 (1.52–5.35); infant aRR 9.18 (1.89–47.88); CNS abnormalities 8% vs 0%
Oropouche virus, spread mainly by biting midges, has caused large outbreaks across Latin America since 2024. Case reports raised concern about vertical transmission; this prospective cohort, published in The Lancet Infectious Diseases on 19 October, provides the first comparison with uninfected pregnancies.
Researchers enrolled 544 pregnant women in outbreak-affected parts of Rio de Janeiro state between late 2024 and October 2025, testing blood, urine, saliva and placenta. 130 had confirmed infection and 271 had none. Adverse maternal outcomes occurred in 18% of infected women against 7% of uninfected (aRR 2.90, 95% credible interval 1.52–5.35), with fetal loss (aRR 3.08) and haemorrhage (aRR 2.79) each about three times as common. Adverse infant outcomes were also more frequent (aRR 9.18, 1.89–47.88), and 8% of infants of infected mothers had central nervous system abnormalities, against none of the uninfected. Having symptoms did not predict outcome: 62% were symptomatic.
The numbers are small and the infant estimate is very imprecise, but the direction is consistent with case reports. Oropouche is not known to circulate in India. The practical relevance is the pregnant traveller, or a woman who conceives soon after return from an affected area.
- Ask pregnant women, and women planning pregnancy, about recent travel to the Amazon basin, Brazil, Peru, Bolivia, Colombia or Cuba.
- In a pregnant woman with fever after such travel, consider Oropouche alongside dengue, Zika and chikungunya, and seek specialist virology advice.
- Advise pregnant women to consider deferring travel to areas with active Oropouche transmission, and to use insect protection if they go.
- An exposed pregnancy without symptoms may still be affected; consider detailed fetal imaging and paediatric follow-up of the infant.
Why it matters
Asymptomatic infection carried the same signal, so symptom-based screening of exposed pregnancies will miss cases.
Don't overread it
One observational cohort in an outbreak setting; it does not establish how often infection harms the fetus elsewhere.
The statistics, in plain English
These are adjusted relative risks with Bayesian credible intervals, read like confidence intervals. The infant outcome estimate (9.18) runs from about 2 to 48, meaning a real increase is likely but its size is very uncertain.
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