More than half of monkeypox cases in the current outbreak may have been passed to others before symptoms appeared, according to a new modelling study from the United Kingdom.
But that's not what epidemiologists found when they investigated contact tracing records from the current monkeypox outbreak in the U.K. As in the U.S., about 95 per cent of recent monkeypox cases in the U.
K. have been among men who have sex with other men. Most cases have been reported after close sexual contact. For the study, researchers culled records on 2,746 monkeypox cases in the U.K. identified from the start of their recent outbreak through August 1. From that larger set of records, they looked for cases with linked contacts where both people had infections that were confirmed by PCR tests and had recorded dates for the start of their symptoms.From these records, they were able to determine a metric called the serial interval, which is roughly the time between the start of symptoms in a case to the start of symptoms in the person they infected.people who filled out questionnaires, researchers were able to pinpoint when they were exposed and when their symptoms first began to calculate the incubation period for the infection -- how long it takes for symptoms to develop after an exposure. They found that the incubation period was sometimes longer than the window between the start of symptoms in a case and their linked contact -- a pattern that's explained when transmission occurs ahead of symptoms. Overall, after researchers adjusted their data to account for possible sources of bias, they found the median serial interval between cases and contacts in the study was shorter than the median incubation period for infections, "which indicates considerably greater pre-symptomatic transmission than previously thought," the study authors write. CNN reached out to the CDC with questions about whether the study might change its guidance on monkeypox, but didn't receive a reply by deadline. The CDC doesn't typically comment on research it is not involved in, and public health agencies don't normally change their advice on the basis of a single study.The researchers estimate that based on their data, more than half of transmission in the U.K. outbreak occurred in this pre-symptomatic phase of the infections. In the study, researchers found transmission occurred up to four days before a person got their first symptoms -- typically a headache, fever, swollen lymph nodes, muscle aches and a rash. Proctitis, a painful swelling of the lining of the rectum, can also occur. "I think that it should change the messaging," said Hanage. "I think that the messaging should be that if you were worried about monkeypox, you cannot assume that your partner is not infectious just because they don't have symptoms." If they're vaccinated, that's a different story, Hanage said, though it's not yet known how effective the vaccines have been at preventing infections. Early in the summer, when vaccine supplies were scarce, public health officials were limiting vaccinations to the known contacts of people with diagnosed with monkeypox, a strategy which likely allowed continued growth of the outbreak because of pre-symptomatic spread, said Swartzberg, who was not involved in the study. Since vaccine doses have become more plentiful, both the U.S. and U.K. have switched to vaccinating people at high risk for catching the infection, which was the right strategy to curtail asymptomatic spread, Swartzberg said. "There is sufficient data now, that shows that monkeypox can also be spread by people with no symptoms and therefore anybody at risk for contracting monkeypox -- whether or not they have symptoms -- should do two things: One is get vaccinated if they haven't been vaccinated, and two is take all necessary precautions to prevent transmitting this virus," Swartzberg said. Other experts say that although the research appears to be well done, it's still just a single study and needs to be repeated by others, hopefully quickly. "This needs confirmation by more studies but has implications for vaccination-based disease elimination strategies which should be seriously considered," said Dr. Boghuma Kabisen Titanji, who is an infectious disease specialist at Emory University. "What proportion of cases are asymptomatic and how much do these cases contribute to seeding new transmission chains? These are urgent questions that need answers," Titanji told the nonprofit Science Media Centre, in a statement about the study.
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