Monkey pox can spread before symptoms appear, study suggests

Monkey pox can spread before symptoms appear, study suggests


More than half of monkeypox cases in the current outbreak may have been transmitted to others before symptoms appeared, according to a new modeling study from the United Kingdom.

The study, led by disease modelers at the UK Health Safety Agency, is at odds with current public health guidance on how monkeypox is spread. It also has important implications for containing outbreaks of infection, particularly those that occur within sexual networks. The research was published in the medical journal The BMJ.

Bill Hanage, an epidemiologist at the Harvard TH Chan School of Public Health, says the new study gets to the heart of a question public health officials have been trying to answer for months: How exactly does the virus spread?

In countries where the virus routinely spreads, such as the Democratic Republic of Congo, most of them who gets it are children living in rural hunting villages. Much of what we know about the virus comes from those settings, where transmission occurs within families living in close proximity. In countries that now have epidemics due to imported cases, almost all of those infected are men who have sex with men, and the routes of transmission have changed.

“The thing with poxviruses in general is, you tend to see transmission when symptoms develop,” said Hanage, who was not involved in the study. “For several months there has been a concern or a growing realization that if you’re talking about transmission through sexual networks, if there’s any kind of contact where pre-symptomatic transmission would be possible, that’s it.”

Hanage says that transmission before people know they are infected helps explain the explosive growth of the epidemic before vaccination became widespread. It also suggests that the virus is likely to be transmitted sexually before a person has symptoms of which they are aware. The monkeypox virus has previously been detected in semen, as well as in anal swabs of infected men who had no symptoms.

Before this study, doctors were aware that it was possible to shed the virus before symptoms appeared, “but we didn’t know how common that was,” said Dr. John Swartzberg, an infectious disease specialist at the University of California at Berkeley School of Health.

As of May 2022, more than 75,000 Monkeypox cases have been reported worldwide, with 99% of those in countries where the virus does not normally spread, according to data from the US Centers for Disease Control and Prevention.

Cases of monkeypox have dropped drastically in the United States of America. As of Oct. 26, the seven-day average of new monkeypox cases reported to the CDC is about 30 per day, down from a high of 446 cases per day in early August.

As of Wednesday, 28,492 cases of monkeypox had been diagnosed in the U.S. According to the CDC.

This progress shows that public health efforts to raise awareness of the infection and encourage vaccination of those at high risk are paying off.

But even when these efforts pay off, public health officials say it’s important to guard against complacency and misinformation about the disease so it doesn’t return.

Currently, official guidelines say that people can transmit the infection only after they develop symptoms.

As of Wednesday, the CDC Web pagefor example, it advises readers that “A person with monkeypox can spread it to others from the time symptoms begin until the rash has completely healed and a fresh layer of skin has formed.”

That instruction is in accordance with information both from the World Health Organization and from Government of Great Britain about how monkeypox spreads.

But that’s not what epidemiologists found when they examined contact tracing records from the current monkeypox outbreak in the UK. As in the US, around 95% of recent cases of monkeypox in the UK have been among men who have sex with other men. Most cases are reported after close sexual contact.

For the study, researchers collected records of 2,746 monkeypox cases in the UK identified from the start of their recent outbreak to 1 August. From that larger set of records, they searched for cases with linked contacts where both individuals had PCR-confirmed infections and had recorded dates of onset of their symptoms.

They found 79 pairs of related cases and contacts who had all the necessary information.

From these records, they were able to determine a metric called the serial interval, which is roughly the time between the onset of symptoms in a case and the onset of symptoms in the person they infected.

From a separate subset of 54 people who filled out questionnaires, the 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 exposure.

They found that the incubation period was sometimes longer than the interval between the onset of symptoms in a case and their associated contact—a pattern explained when transmission occurs before symptoms.

Overall, after the researchers adjusted their data to account for possible sources of bias, they found that the mean serial interval between cases and contacts in the study was shorter than the mean incubation period for infections, “suggesting a significantly 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 guidelines on monkeypox, but did not receive a response by deadline. The CDC doesn’t usually comment on research it’s not involved in, and public health agencies don’t usually change their advice based on a single study.

The researchers estimate that, based on their data, more than half (53%) of UK transmissions occurred in this pre-symptomatic stage of infection.

In the study, researchers found that transmission occurred up to four days before a person developed the first symptoms — typically headache, fever, swollen lymph nodes, muscle aches and a rash. Proctitis, a painful swelling of the rectal mucosa, may also occur.

“I think it should change the messaging,” Hanage said. “I think the message should be that if you’re worried about monkeypox, you can’t assume your partner isn’t contagious just because they don’t have symptoms.”

If they’ve been vaccinated, that’s a different story, Hanage said, although it’s not yet known how effective the vaccines have been in preventing infections.

In early summer, when vaccine supplies were scarce, public health officials limited vaccination to known contacts of people diagnosed with monkeypox, a strategy that likely allowed the outbreak to continue due to presymptomatic spread, said Swartzberg, who was not involved in the study.

As vaccine doses became higher, both the U.S. and U.K. switched to vaccinating people at high risk of infection, which was the right strategy to curb asymptomatic spread, Swartzberg said.

“There is now enough data to show that monkeypox can be transmitted by people without symptoms and therefore anyone at risk of contracting monkeypox – whether they have symptoms or not – should do two things: one is to get vaccinated if she has not been vaccinated, and two are taking all the necessary precautions to prevent the transmission of this virus,” said Swartzberg.

Other experts say that while the research appears to be well done, it’s still just one study and needs to be replicated by others, hopefully soon.

“This needs to be confirmed by more studies, but has implications for vaccine-based disease elimination strategies that should be seriously considered,” said Dr. Boghuma Kabisen Titanji, an infectious disease specialist at Emory University.

“What proportion of cases are asymptomatic and how much do these cases contribute to the creation of new chains of transmission? These are urgent questions that need answers,” Titanji told the nonprofit Science Media Center, in a statement about the study.

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