Monkeypox can spread before symptoms start, study finds


According to a new modeling study from the UK, more than half of monkeypox cases in the current outbreak may have been passed on to others before symptoms appeared.

The study, which was led by disease modellers from the UK Health Security Agency, is contrary to current public health guidance on the spread of monkeypox. It also has important implications for how to contain outbreaks of infection, especially those that arise within sexual networks. The research is published in the medical journal The BMJ.

Bill Hanage, an epidemiologist at 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- does he?

In countries where the virus spreads regularly, such as the Democratic Republic of Congo, most of those who catch it are children living in rural hunting villages. Much of what we know about the virus comes from these settings, where transmission occurs in families living in close quarters. In countries currently experiencing outbreaks due to imported cases, almost all infected people are men who have sex with men, and the routes of transmission have changed.

“The problem is that with poxviruses in general, you tend to see transmission once symptoms develop,” said Hanage, who was not involved in the study. “For several months now there’s been a concern, or some sort of growing awareness, that if you’re talking about transmission in sexual networks, if there’s any type of contact where pre-symptomatic transmission would be possible, that’s all.”

Hanage says transmission before people knew they were infected helps explain the explosive growth of the epidemic before vaccination became widespread. It also suggests that the virus is likely transmitted sexually before a person has symptoms of which they are aware. The Monkeypox virus has already been detected in semen, as well as in anal swabs from infected men who showed no symptoms.

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

Since May 2022, more than 75,000 cases of monkeypox have been reported worldwide, 99% of them in countries where the virus is not commonly spread, according to data from the US Centers for Disease Control and Prevention.

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

There were 28,492 cases of monkeypox diagnosed in the United States as of Wednesday, according to the CDC.

These advances indicate that public health efforts to raise awareness of the infection and encourage vaccination of those at high risk are working.

But even if those efforts pay off, public health officials say it’s important to guard against complacency and misinformation about the disease to keep it from coming back.

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

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

These guidelines align with information from the World Health Organization and the UK government on the spread of monkeypox.

But that’s not what epidemiologists found when they investigated 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 are in men who have sex with men. Most cases have been reported after close sexual contact.

For the study, the researchers selected records from 2,746 cases of monkeypox in the UK identified from the start of their recent outbreak until August 1. From this larger set of records, they looked for cases with linked contacts where both people had PCR-confirmed infections. tests and had recorded dates for the onset of their symptoms.

They found 79 pairs of linked cases and contacts that had all the required information.

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

From a distinct subset of 54 people who completed questionnaires, researchers were able to determine when they were exposed and when their symptoms began to calculate the incubation period of infection – how long it takes for symptoms to develop after exposure.

They found that the incubation period was sometimes longer than the window between the onset of symptoms in a case and their linked contact – a pattern that is explained when transmission occurs before symptoms.

Overall, after the researchers adjusted their data for possible sources of bias, they found that the median serial interval between cases and contacts in the study was shorter than the time period. median incubation of infections, “indicating significantly greater pre-symptomatic transmission than previously thought,” the study authors write.

CNN contacted the CDC to ask if the study might change its directions on monkeypox, but did not receive a timely response. The CDC typically doesn’t comment on research it isn’t involved in, and public health agencies don’t normally change their advice based on a single study.

The researchers estimate that, based on their data, more than half (53%) of transmission during the UK outbreak occurred in this pre-symptomatic phase of infections.

In the study, the researchers found that transmission occurs up to four days before a person shows their first symptoms – usually a headache, fever, swollen lymph nodes, muscle aches and skin rash. Proctitis, a painful swelling of the lining of the rectum, may also occur.

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

If they get 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 vaccinations to known contacts of people diagnosed with monkeypox, a strategy that likely allowed the epidemic to continue growing due to pre-symptomatic spread, said Swartzberg, who was not involved in the study.

Since vaccine doses became more plentiful, the United States and the United Kingdom switched to vaccinating people at high risk of contracting the infection, which was the right strategy to limit asymptomatic spread, said Swartzberg.

“There is now enough data to show that monkeypox can also be transmitted by people who have no symptoms. Therefore, anyone at risk of contracting monkeypox – whether or not they have symptoms – should do two things: one is to get vaccinated if she hasn’t been vaccinated, and two take all necessary precautions to avoid transmitting this virus,” Swartzberg said.

Other experts say that although the research appears to be well done, it is still just one study and needs to be repeated by others, hopefully soon.

“This needs to be confirmed by further 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 the University. Emory.

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

Leave a Comment