The mpox virus is normally endemic to Africa but has recently been found on other continents. It spreads through prolonged, direct contact with infected people or their bedding, clothing and towels.
September 6, 2022 - By Krista Conger
Note: This article was originally published June 10, 2022, and was updated Sept. 6, 2022.
In May, the first reports about mpox circulating in Europe began to surface. Now, several months later, thousands of cases have been identified around the globe, in countries where the virus is not usually found. On July 23, the World Health Organization declared the outbreak “a public health emergency of international concern.” New York City and San Francisco, as well as New York state, California and Illinois, soon declared their own emergencies and, on Aug. 4, the U.S. government declared the spread of monkeypox a national public health emergency. In the face of a global COVID-19 pandemic, it would be easy to view these developments with anxiety. But the mpox virus differs in several ways from the virus that causes COVID-19, limiting its ability to cause similar chaos and loss of life.
What is mpox and where is it usually found?
The mpox virus, formerly known as the monkeypox virus, is related to the viruses that cause smallpox and cowpox. Each of these diseases causes a characteristic skin rash that can be accompanied by fever, head and muscle aches, swollen lymph nodes, and exhaustion. The mpox virus, which was first found in monkeys used for research in Denmark in 1958, is not new to humans: The first patient with mpox was reported in central Africa in 1970. Since then, the virus has mainly infected people in central and western Africa, most often after they come into contact with an infected animal — usually squirrels, rodents and monkeys that live in tropical rainforests.
Until recently, transfer of the virus between humans was thought to be relatively uncommon; the longest documented chain of infection from one person to another had involved six people. The mpox virus spreads from one person to another primarily through bodily fluids including saliva and semen; close skin-to-skin contact; or prolonged exposures to large respiratory droplets or contaminated materials, such as an infected person’s bedding.
“There are two related types, or clades, of monkeypox virus,” said Stanley Deresinski, MD, clinical professor of infectious disease at Stanford Medicine. “The clade originating in central Africa has a mortality rate of about 10%, while the clade from west Africa, which is the source of the current outbreak, has a mortality rate of about 1%. Mortality is also presumably related to host factors, including whether an infected person is malnourished or immunocompromised.”
In mid-August, the WHO announced a name change for the clades to avoid promoting negative perceptions of cultural, social or ethnic groups, or negatively affecting trade, travel, tourism or animal welfare. The west African clade has been renamed to Clade II, and the central African clade is now Clade I. The organization also renamed the virus mpox to replace monkeypox.
What’s behind the rise in cases we are seeing now? How many countries are affected?
Experts are not sure why the virus has begun to spread between people outside of Africa. As of late August, mpox has been identified in 98 countries, 91 of which had not previously reported cases of mpox. All 50 states in the U.S., as well as Puerto Rico and the District of Columbia, have reported infections — nearly 17,000 patients with mpox since the first case of human transmission was reported in the United Kingdom in early May. Nearly 47,000 cases worldwide have now been identified, almost all in people who haven’t traveled to Africa and who haven’t had contact with an infected animal. Although most of the people who have been infected are men who have sex with men, anyone who comes into close physical contact with an infected person or with their clothing, bedding or towels can be infected with mpox. (For more information about transmission without sexual contact, see this article.) As of Aug. 22, the WHO has reported 12 deaths associated with infection in the current outbreak.
What are the symptoms? Is there a treatment?
“Monkeypox classically manifests first with a fever, swollen lymph nodes, malaise and headache for a few days, and that is followed by a rash,” said Jorge Salinas, MD, assistant professor of infectious disease and co-medical director of Stanford Medicine’s Infection Prevention and Control Program. “People can develop lesions that can have pus, similar to pimples. They can grow larger than a regular pimple, and some people can have a few lesions or multiple lesions. Depending on where the lesions occur, the symptoms can be mild or more severe. Some people have lesions in their genital areas, for example, and those tend to be more painful and problematic.”
According to the Centers for Disease Control and Prevention, most infected people will have a mild course of the disease over two to four weeks. The time between exposure to an infected person and the development of the first symptoms can vary between five and 21 days but is typically between one and two weeks. People with mild cases can usually recover at home. They should isolate themselves from others until the rash has scabbed over and new skin has formed. If they must be in contact with others, they should cover the rash with clothing or bandages and wear a well-fitting mask covering their mouth and nose.
People with severe disease — or with extensive lesions or lesions near or in the eyes, mouth, urethra or rectum — or those who are at risk for developing severe disease can be treated with an oral antiviral medication called tecovirimat, or Tpoxx, that was developed to treat people with smallpox. Additionally, in 2019, the U.S. Food and Drug Administration approved a vaccine called Jynneos against smallpox and mpox in people 18 and over. Vaccinating people known to have recently been in close contact with an individual infected with mpox may protect from infection or limit the severity of the disease, according to the CDC.
Ironically, the success of global vaccination campaigns and the subsequent eradication of naturally occurring smallpox infections mean that many younger people living today have not been vaccinated against smallpox. As a result, mpox cases have been increasing in Africa during the past several decades.
How concerned should people be?
“Unlike the virus that causes COVID-19, the monkeypox virus is not known to be transmitted over a distance by small aerosol particles, although it is possible that it may spread through respiratory secretions when people have close face-to-face contact,” Deresinski said.
In general, people should be cautious but not fearful, Deresinski added. “Everyone is susceptible to monkeypox, but becoming infected involves prolonged close contact with another person or contaminated objects. It’s not like COVID-19. Furthermore, most people with monkeypox are readily recognized as ill even before a rash arises.”
Is there more than one vaccine against mpox? Do they leave a scar?
In addition to Jynneos, there is a vaccine called ACAM2000 that has been used since 2007 to protect select groups — researchers working with highly dangerous viruses and some members of the military, for example — against smallpox. Both vaccines use a live version of a pox-like virus called vaccinia to promote the development of an immune response to the pox virus family. But the virus in the Jynneos vaccine, although live, has been altered so it can’t replicate in human cells. In contrast, the vaccinia virus in ACAM2000 can replicate in cells, and care must be taken to cover the vaccination site to avoid spreading the vaccinia virus to other places on the body or to other people.
ACAM2000 is given as a single dose, and immune protection peaks 28 days after vaccination. People with weakened immune systems, including those with HIV or AIDS, or those with skin conditions such as eczema, dermatitis or psoriasis, should consult their health care provider before receiving ACAM2000. This vaccine often leaves a small scar.
The Jynneos vaccine is given as two doses 28 days apart, and peak protection is conferred two weeks after the second dose; it rarely leaves a scar. A 2015 study showed that when the vaccine is administered intradermally (between the skin layers), one-fifth the dose stimulates the same immune responses as a full dose given subcutaneously (under the skin). On Aug. 9 the FDA issued an emergency use authorization allowing health care providers to vaccinate people intradermally with the smaller dose as a way to increase the number of people who can be vaccinated with the current supply.
ACAM2000 is estimated to be about 95% effective in preventing smallpox infection and about 85% effective against monkeypox, but it has the potential for more severe side effects, including serious cardiac complications, than the Jynneos vaccine, which is also estimated to be about 85% effective against monkeypox. “The side effect profile of ACAM2000 was tolerable when used to prevent smallpox because smallpox has a fatality rate of about 30%,” professor of pediatrics and of epidemiology and population health Yvonne Maldonado, MD, said. “But monkeypox is less lethal than smallpox, and we now have another option that is better tolerated and doesn’t use replicating virus.”
How protected are people after just one dose of the Jynneos vaccine?
Although there are no firm data on the protection conferred by a single shot of the Jynneos vaccine, some experts feel that it may lead to at least partial or temporary protection and that such a strategy is warranted during a public health emergency. But, as with most vaccines, breakthrough cases will occur even in fully vaccinated people.
“Personally, I would not assume the first dose alone is protective,” Maldonado said. “I would be very careful.”
The United Kingdom is prioritizing the first dose for people at high risk and will offer a second dose after two to three months as more vaccine becomes available.
How much of the Jynneos vaccine does the United States have? How many people have been vaccinated?
By mid-August, the U.S. government had provided nearly 1 million doses of the Jynneos vaccine. On Aug. 18 the White House announced the upcoming availability of an additional 1.8 million doses (based on intradermal injection volumes). The company continues to make more vaccine.
“About 1.6 million people in the United States are considered to be at high risk,” Maldonado said. “So with a two-dose regimen, we will need at least 3.2 million doses.”
What’s being done to understand this outbreak and how to stop it?
The CDC and the World Health Organization are tracking the spread of cases and coordinating with health care workers to stem the transmission of the virus by isolating patients and identifying close contacts who may benefit from antiviral treatment or vaccination. The CDC has released a fact sheet for people who are sexually active to learn the symptoms of the disease and ways to prevent infection. “As cases are identified, ring vaccination strategies, or vaccination of contacts before they become ill, are implemented,” Deresinski said.
The emergency declarations by the U.S. and the WHO may sound dire, but they do not mean that the spread of the virus is unstoppable, experts say. In fact, there are recent signs that the pace of infections may be slowing. The declarations are necessary to facilitate many steps essential to slowing or halting the outbreak, including increased access to grant funding for research, the easier sharing of data among government organizations, and the coordination of federal and international responses.
What about pregnant people and children?
Although the current outbreak in Europe and North America primarily involves men who have sex with men, data from Africa show that others can be infected with the virus, including children and pregnant people. According to the WHO, infection during pregnancy has not been well studied but may harm the fetus.
“We know that this disease can be more severe in young children and people who are immune compromised,” Salinas said.
“Overall, my concern for children acquiring monkeypox is low, unless the child had been in close contact with someone diagnosed with monkeypox,” said Roshni Mathew, MD, a clinical associate professor in pediatric infectious diseases. “Children frequently have rash, and pediatricians are skilled at diagnosing the various causes. A childhood viral illness that could appear similar to monkeypox is chickenpox. But there are characteristic differences between the two. Chickenpox lesions, for example, are superficial, erupt in crops, and evolve quickly and independently, while monkeypox lesions are deep-seated, well-circumscribed, and relatively the same size and stage of development as other nearby lesions.”
What proportion of people infected with monkeypox are men who have sex with men?
The WHO reported on Aug. 22 that more than 98% of cases in which the gender of the patient was known were in men, and nearly 96% of cases in which sexual orientation was known were in men who have sex with men. 82% of all cases were linked to probable sexual transmission, and 45% of cases were in people who are HIV-positive. A report from the CDC covering the period from May 17, when the first U.S. case was diagnosed, to July 22 mirrors these numbers, finding that 99% of cases were in men and that 94% of these reported recent sexual or intimate contact with another man.
The cases in the U.S. also show a disproportionate impact among people of color. Black and Hispanic people, who make up about one-third of the population, accounted for just over half of all cases, while white people account for 41% of the cases. For cases whose HIV status was known, 41% were HIV positive.
The unequal burden of mpox in gay and bisexual men and in marginalized ethnic groups can create challenges for effective public health messaging and equitable access to vaccines and treatments for mpox.
“There is a high intersectionality with BIPOC [Black, Indigenous and people of color] groups, which have historically been subjected to medical malpractice and even experimentation,” said Benjamin Laniakea, MD, clinical assistant professor of primary care and population health. “It’s important for doctors and public health experts to consider how to communicate thoughtfully and effectively with these populations.”
If I am in a high-risk group, how can I best protect myself while remaining sexually active?
The first and most effective way to protect yourself is to get vaccinated. Closing off your sexual networks can be another way to reduce your risk of exposure, as well as asking sexual or intimate partners whether they have any sores or have been in close contact with people diagnosed with mpox.
“Be judicious about your sexual circles, and if you’re meeting someone for the first time, consider exchanging contact information so you can let each other know if you develop symptoms after your encounter,” Laniakea said. “Make sure you stay on top of protective measures for other sexually transmitted diseases. If you are on PRrEP [medication that protects against HIV infection], continue that. We don’t know if condoms prevent the spread of monkeypox, but they can block other infections.”
People who share sex toys should use condoms with them, and change the condom between recipients, according to Laniakea, because it is not clear how best to clean the toys.
What should I do if I have been exposed to someone with mpox? If I am diagnosed with mpox, how long must I wait before resuming sexual activity?
People with a known exposure to mpox should contact their health care provider as soon as possible to ask about prophylactic vaccination or Tpoxx, which can prevent infection or reduce symptoms if started within a few days of exposure. If you are diagnosed with mpox, refrain from sexual contact until any lesions have completely scabbed over and the scab has fallen off to reveal healthy, intact skin underneath. “An open sore under the scab doesn’t count,” Laniakea said. “The scab could have been rubbed off through friction against clothing, or from picking. If the skin underneath is not completely healed, you are still likely to be infectious.”
Does a childhood smallpox vaccination protect against mpox?
The short answer is … maybe. But like with other vaccines, protection seems to decline over time. In a study in Spain, 32 of 181 mpox patients reported being vaccinated as a child against smallpox. But experts caution it’s not possible to know whether the previous vaccination provided partial protection, or if it reduced the severity of the course of the disease in the people who did become infected. Another study in 2020 found that HIV infection may cause the protection from a childhood smallpox vaccination to wane more quickly.
Regardless, routine smallpox vaccination was halted in the United States in 1971 as the number of smallpox cases dwindled globally. Some people without access to their childhood vaccination records rely on the presence of the distinctive scar on their upper arm to infer whether they have received a smallpox vaccination. But the scars can be difficult to see. They are also similar to another scar left by a vaccine called BCG that protects against tuberculosis. This vaccine is still administered routinely to babies in many countries outside the U.S., including Mexico, and the scar can be confused with the scar from the smallpox vaccine. If you were born in the U.S. and are younger than 50, it is unlikely that you received a smallpox vaccination as a child.
Will people vaccinated with Jynneos or ACAM2000 need a booster shot?
Recipients of either vaccine will likely need a booster after a few years to receive ongoing protection. If you’re in a high-risk group, it’s unwise to depend on a childhood smallpox vaccination to protect against mpox during the current outbreak.
“The main thing you can do to protect yourself is to be familiar what the risk factors are, seek out a vaccination if you are in a high-risk group and try to limit risky behaviors,” Maldonado said. “We are really lucky we have this vaccine available.”
If you or someone you know develops an unexplained rash that progresses to blisters and was preceded by a fever, headache and swollen lymph nodes, please consult a health care professional. Patients who want more information can visit Stanford Medicine’s web page about mpox. Health care workers can view the mpox resource page.
About Stanford Medicine
Stanford Medicine is an integrated academic health system comprising the Stanford School of Medicine and adult and pediatric health care delivery systems. Together, they harness the full potential of biomedicine through collaborative research, education and clinical care for patients. For more information, please visit med.stanford.edu.