When properly administered before an exposure, vaccines are effective at protecting people against monkeypox.

ACAM2000 and JYNNEOSTM (also known as Imvamune or Imvanex) are the two currently licensed vaccines in the United States to prevent smallpox.

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Pre-Exposure Prophylaxis To Prevent Monkeypox

The Advisory Committee on Immunization Practices (ACIP) recommends that people whose jobs may expose them to orthopoxviruses, such as monkeypox, get vaccinated with either ACAM2000 or JYNNEOS to protect them if they are exposed to an orthopoxvirus. This is known as pre-exposure prophylaxis (PrEP). People who should get PrEP include:

  • Clinical laboratory personnel who perform testing to diagnose orthopoxviruses, including those who use polymerase chain reaction (PCR) assays for diagnosis of orthopoxviruses, including Monkeypox virus
  • Research laboratory workers who directly handle cultures or animals contaminated or infected with orthopoxviruses that infect humans, including Monkeypox virus, replication-competent Vaccinia virus, or recombinant Vaccinia viruses derived from replication-competent Vaccinia virus strains
  • Certain healthcare and public health response team members designated by public health authorities to be vaccinated for preparedness purposes

People who can get PrEP if they want to receive it include healthcare personnel who administer ACAM2000 or anticipate caring for many patients with monkeypox.

At this time, most clinicians in the United States and laboratorians not performing the orthopoxvirus generic test to diagnose orthopoxviruses, including monkeypox, are not advised to receive orthopoxvirus PrEP. Laboratorians should consult with laboratory biosafety officers and supervisors to identify risks and precautions, depending on the type of work they are doing. Regardless of whether they get PrEP, clinicians and laboratorians should use recommended infection control practices; these are important to preventing any infection.

ACAM2000 is administered as a live Vaccinia virus preparation that is inoculated into the skin by pricking the skin surface. Following a successful inoculation, a lesion will develop at the site of the vaccination (i.e., a “take”). The virus growing at the site of this inoculation lesion can be spread to other parts of the body or even to other people. Individuals who receive vaccination with ACAM2000 must take precautions to prevent the spread of the vaccine virus and are considered vaccinated within 28 days.

JYNNEOSTM is administered as a live virus that is non-replicating. It is administered as two subcutaneous injections four weeks apart. There is no visible “take” and as a result, no risk for spread to other parts of the body or other people. People who receive JYNNEOS TM are not considered vaccinated until 2 weeks after they receive the second dose of the vaccine.

CDC, in conjunction with the Advisory Committee on Immunization Practices (ACIP), provides recommendations on who should receive smallpox vaccination in a non-emergency setting. At this time, vaccination with ACAM2000 is recommended for laboratorians working with certain orthopoxviruses and military personnel. On November 3, 2021, ACIP voted to recommend JYNNEOS pre-exposure prophylaxis as an alternative to ACAM2000 for certain persons at risk for exposure to orthopoxviruses.

Vaccine Effectiveness

Because Monkeypox virus is closely related to the virus that causes smallpox, the smallpox vaccine can protect people from getting monkeypox. Past data from Africa suggests that the smallpox vaccine is at least 85% effective in preventing monkeypox. The effectiveness of JYNNEOSTM against monkeypox was concluded from a clinical study on the immunogenicity of JYNNEOS and efficacy data from animal studies.

Smallpox and monkeypox vaccines are effective at protecting people against monkeypox when given before exposure to monkeypox. Experts also believe that vaccination after a monkeypox exposure may help prevent the disease or make it less severe.

Receiving Vaccine After Exposure to Monkeypox Virus

The sooner an exposed person gets the vaccine, the better.

CDC recommends that the vaccine be given within 4 days from the date of exposure in order to prevent onset of the disease. If given between 4–14 days after the date of exposure, vaccination may reduce the symptoms of disease, but may not prevent the disease.

Clinician demonstrates the use of a bifurcated needle during the 2002 Smallpox Vaccinator Workshop.

Smallpox and monkeypox vaccines are effective at protecting people against monkeypox when given before exposure to monkeypox. Experts also believe that vaccination after a monkeypox exposure may help prevent the disease or make it less severe.

Revaccination After Exposure

Persons exposed to monkeypox virus and who have not received the smallpox vaccine within the last 3 years, should consider getting vaccinated.

The sooner the person receives the vaccine, the more effective it will be in protecting against monkeypox virus.

Vaccine Risks vs. Monkeypox Disease

For most persons who have been exposed to monkeypox, the risks from monkeypox disease are greater than the risks from the smallpox or monkeypox vaccine.

Monkeypox is a serious disease. It causes fever, headache, muscle aches, backache, swollen lymph nodes, a general feeling of discomfort, exhaustion, and severe rash. Studies of monkeypox in Central Africa—where people live in remote areas and are medically underserved—showed that the disease killed up to 11% of people infected.

In contrast, most people who get the smallpox or monkeypox vaccine have only minor reactions, like mild fever, tiredness, swollen glands, and redness and itching at the place where the vaccine is given. However, these vaccines do have more serious risks, too.

In certain groups of people, such as people with serious immune system problems, complications from ACAM2000 can be severe. If you have concerns about whether you should receive ACAM2000, talk to your healthcare provider. This vaccine has the potential for more side effects and adverse events than the newer vaccine, JYNNEOS.

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