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August 11, 2022

What we know (and don't know) about the spread of monkeypox

Daily Briefing

    As the global monkeypox outbreak continues to grow, evidence indicates that the virus is largely spreading through direct skin contact with lesions caused by the infection. However, new research suggests that the virus could also spread through other means, such as bodily fluids and asymptomatic individuals.

    Monkeypox is officially a public health emergency

    How monkeypox is spreading in the current outbreak

    The current monkeypox outbreak, which initially began in mid-May, has resulted in nearly 30,000 cases worldwide, with around 9,500 cases in the United States alone. Since then, both the World Health Organization (WHO) and the United States have declared the outbreak a public health emergency.

    However, despite the monkeypox's unprecedented spread during the current outbreak, infectious disease experts say that the virus's methods of transmission have remained largely the same as outbreaks of the past.

    "This virus didn't get a makeover," said Peter Chin-Hong, an infectious disease physician at University of California- San Francisco. "The transmission we're seeing is completely consistent with what we've known about monkeypox, just in a new context."

    According to STAT News, there are three ways exposure to the virus can result in infection:

    1. Direct skin contact with lesions caused by the virus
    2. Contact with contaminated surfaces
    3. Contact with respiratory secretions, such as saliva, from someone who has a lesion in their mouth or throat

    So far, monkeypox's primary mode of transmission seems to be through direct skin contact with lesions.  Brief contact is unlikely to facilitate spread; repeated touching or rubbing is likely required. Although it is not a sexually transmitted disease, most cases in this current outbreak have occurred through sexual contact, particularly among men who have sex with men (MSM).

    According to a recent WHO report, 91% of more than 5,000 global monkeypox cases were attributed to a sexual encounter. "Up until this point in time, the 2022 multi-country monkeypox outbreak has been overwhelmingly concentrated in MSM networks," WHO officials said.

    Other studies suggest that monkeypox may also be spreading through semen and other bodily fluids during sexual contact. For example, a recent study published in the New England Journal of Medicine found that the virus was present in 29 out of 32 samples collected from monkeypox patients.

    Similarly, researchers from the Hospital Clinic de Barcelona found high viral loads in the semen of seven out of nine patients with monkeypox and were able to detect the virus in urine, feces, and blood in the majority of samples from 12 other patients.

    "I would be very surprised if the virus is not infectious in some samples with high viral loads," said Miguel Martinez, an associate professor of microbiology who helped lead the Barcelona study. "There is a wide distribution of the virus in bodily fluids at least in the acute phase of the illness, and I think this is contributing to spread of the disease."

    In comparison, transmission through other means, including contaminated surfaces or objects, has been much lower. Based on WHO's report, only around 0.2% of monkeypox cases have been due to contact with contaminated items.

    And despite recent concerns about the potential of airborne transmission, there has been little evidence of it occurring. Contact tracing studies from Europe suggest that it is "an unlikely source of significant spread," since household transmission has been limited, STAT News reports.

    "If anyone was going to get infected by shared air, it would be household contacts," Chin-Hong said. "That's a compelling piece of data that shows even in these enriched household environments, the virus is not transmitting efficiently."

    Can asymptomatic patients spread the virus?

    Currently, it is not known whether monkeypox can be transmitted by asymptomatic patients, but early research suggests it may be a possibility. According to CDC, people without symptoms cannot spread monkeypox.

    But in a preprint study, researchers in Belgium retrospectively tested 224 swab samples from male patients who had visited a sexual health clinic in May and found evidence of monkeypox in three of the samples. None of these three patients reported any symptoms in the weeks before or after the sample was collected, and none of their contacts ever developed monkeypox.

    "This is different to what has previously been understood about monkeypox," said Chloe Orkin, a professor of HIV medicine at Queen Mary University of London. Typically, monkeypox replicates in skin tissue, so it is unusual for an individual to be infected without at least developing some lesions.

    Currently, more research is needed to understand how monkeypox affects asymptomatic individuals and whether there are any potential transmission risks.

    According to Chin-Hong, the recent studies on monkeypox's transmission highlight the need for physicians to reconsider how they diagnose the disease. Usually, physicians swab patients' skin and any visible lesions, but "[t]hat's prehistoric," he said. "We need explicit guidance about swabbing different sites—that's going to be really important for keeping infected people from spreading this."

    Overall, many experts said that the risk of contracting monkeypox is low for most people, but noted that public health officials should be treating the outbreak more seriously.

    "We will not be able to control this epidemic until we have an effective vaccine available that we can use widely, and we are far from that," said José Luis Blanco, an infectious disease researcher at the Hospital Clinic de Barcelona. "This is not an MSM infection, it is an infection that has started affecting MSM but can affect all of us." (Molteni, STAT News, 8/10; Mandavilli, New York Times, 8/2)

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