Overview of Immunization

With rare exceptions, simultaneous administration of vaccines is safe, effective, and convenient; it is particularly recommended when children may be unavailable for future vaccination or when adults require multiple vaccines before international travel. An exception is simultaneous administration of pneumococcal conjugate vaccine and the meningococcal conjugate vaccine MenACWY-D to children with functional or anatomic asplenia; these vaccinations should not be administered during the same visit but should be separated by ≥ 4 weeks. If vaccination against both meningococcal serotype B and serotypes A, C, W, and Y is indicated (previously available only separately), the MenACWY and MenB combination vaccine is recommended (5).

Simultaneous administration may involve combination vaccines (see table Vaccines Available in the United States) or use of ≥ 1 single-antigen vaccines. More than one vaccine may be administered at the same time using different injection sites and syringes.

If live-virus vaccines (eg, varicella, measles-mumps-rubella [MMR]) are not administered at the same time, they should be administered ≥ 4 weeks apart (1).

Many childhood vaccines are recommended for simultaneous administration, particularly in the first year of life. In general, vaccines are designed to be administered according to schedules that optimize their efficacy. Although some patients may have concerns about excessive antigenic exposure, the immune system is exposed to far more antigens in everyday life. (See also Use of Multiple, Simultaneous Vaccines.)

1. 1. National Center for Immunization and Respiratory Diseases. General recommendations on immunization --- recommendations of the Advisory Committee on Immunization Practices (ACIP) [published correction appears in MMWR Recomm Rep. 2011 Jul 29;60:993]. MMWR Recomm Rep. 2011;60(2):1-64

2. 2. Centers for Disease Control and Prevention. Chapter 3: Immunization Strategies for Healthcare Practices and Providers. CDC Pink Book. 2021. Accessed April 17, 2025.

3. 3. National Vaccine Advisory Committee. Recommendations from the National Vaccine Advisory committee: standards for adult immunization practice. Public Health Rep. 2014;129(2):115-123. doi:10.1177/003335491412900203

4. 4. Centers for Disease Control and Prevention. Adult Immunization Standards. 2024. Accessed February 19, 2025.

5. 5. Collins JP, Crowe SJ, Ortega-Sanchez IR, et al. Use of the Pfizer Pentavalent Meningococcal Vaccine Among Persons Aged ≥10 Years: Recommendations of the Advisory Committee on Immunization Practices - United States, 2023. MMWR Morb Mortal Wkly Rep. 2024;73(15):345-350. Published 2024 Apr 18. doi:10.15585/mmwr.mm7315a4

For many vaccines, the only contraindication is a serious allergic reaction (eg, anaphylactic reaction) to the vaccine or to one of its components.

Egg allergy is common in the United States. Some vaccines produced in cell culture systems, including most influenza vaccines, contain trace amounts of egg antigens. CDC guidelines for the influenza vaccine state that patients with a history of egg allergy should be given the influenza vaccine. An egg allergy necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity of previous reaction to egg. Any influenza vaccine that is otherwise recommended based on the recipient’s age and health status can be used. However, severe allergic reactions to egg protein are a contraindication for the yellow fever vaccine (which is administered to travelers to endemic areas). , contain trace amounts of egg antigens. CDC guidelines for the influenza vaccine state that patients with a history of egg allergy should be given the influenza vaccine. An egg allergy necessitates no additional safety measures for influenza vaccination beyond those recommended for any recipient of any vaccine, regardless of severity of previous reaction to egg. Any influenza vaccine that is otherwise recommended based on the recipient’s age and health status can be used. However, severe allergic reactions to egg protein are a contraindication for the yellow fever vaccine (which is administered to travelers to endemic areas).

According to the CDC, a localized or delayed hypersensitivity reaction to thimerosal, a mercury-containing compound, is not a contraindication to receiving a vaccine that includes thimerosal (see CDC: Thimerosal and Vaccines). The use of thimerosal in vaccines has been reduced significantly since the late 1990s, primarily because of public concern about mercury exposure.

Patients who developed Guillain-Barré syndrome (GBS) within 6 weeks after a previous influenza or diphtheria-tetanus-acellular pertussis (DTaP) vaccination may be given the vaccine if the benefits of vaccination are thought to outweigh the risks. For example, for patients who developed the syndrome after a dose of DTaP, clinicians may consider giving them a dose of the vaccine if a pertussis outbreak occurs; however, such decisions should be made in consultation with an infectious disease specialist.

There have been rare reports of GBS after vaccination with certain COVID-19 vaccines, specifically the Ad26.COV2.S vaccine (1). However, the likelihood of developing GBS after vaccination appears to be low compared to the risk of GBS associated with COVID-19 infection itself (2). The Ad26.COV2.S vaccine is no longer available.

The Advisory Committee on Immunization Practices no longer considers a history of GBS to be a contraindication or precaution for use of the meningococcal conjugate vaccine, although it remains listed as a precaution in the prescribing information (see CDC: Meningococcal Vaccines Safety Information).

A significant fever (temperature of > 39° C) or severe illness without fever requires delaying vaccination, but minor infections, such as the common cold (even with low-grade fever), do not. This precaution prevents confusion between the manifestations of the underlying illness and possible adverse effects of the vaccine and prevents superimposition of adverse effects of the vaccine on the underlying illness. Vaccination can be postponed until the illness resolves, if possible.

Pregnancy is a contraindication to vaccination with the MMR vaccine, intranasal influenza vaccine (live), varicella virus vaccine, chikungunya vaccine, yellow fever vaccine, and any other live-virus vaccines. For pregnant patients, MenB is delayed until after pregnancy unless patients are at increased risk and vaccination benefits outweigh potential risks.Pregnancy is a contraindication to vaccination with the MMR vaccine, intranasal influenza vaccine (live), varicella virus vaccine, chikungunya vaccine, yellow fever vaccine, and any other live-virus vaccines. For pregnant patients, MenB is delayed until after pregnancy unless patients are at increased risk and vaccination benefits outweigh potential risks.

The Advisory Committee on Immunization Practices recommends delaying vaccination with human papillomavirus 9-valent vaccine and recombinant zoster vaccine until after pregnancy (see The Advisory Committee on Immunization Practices recommends delaying vaccination with human papillomavirus 9-valent vaccine and recombinant zoster vaccine until after pregnancy (seeAdult Immunization Schedule by Medical Condition and Other Indication).

Immunocompromised patients should, in general, not receive live-microbial vaccines, which could provoke severe or fatal infections. If this is caused by immunosuppressive therapy (eg, high-dose corticosteroids [≥ 20 mg prednisone or equivalent for ≥ 2 weeks], antimetabolites [eg, anti-cancer therapy], immune modulators, alkylating compounds, radiation), live-virus vaccines should be withheld until the immune system recovers after treatment (the interval of time varies depending on the therapy used). Patients taking immune-suppressing medications for any of a wide variety of disorders, including dermatologic, gastrointestinal, rheumatologic, and pulmonary disorders, should not receive live-virus vaccines. For patients receiving long-term immunosuppressive therapy, clinicians should discuss the risks and benefits of vaccination and/or revaccination with an infectious disease specialist.Immunocompromised patients should, in general, not receive live-microbial vaccines, which could provoke severe or fatal infections. If this is caused by immunosuppressive therapy (eg, high-dose corticosteroids [≥ 20 mg prednisone or equivalent for ≥ 2 weeks], antimetabolites [eg, anti-cancer therapy], immune modulators, alkylating compounds, radiation), live-virus vaccines should be withheld until the immune system recovers after treatment (the interval of time varies depending on the therapy used). Patients taking immune-suppressing medications for any of a wide variety of disorders, including dermatologic, gastrointestinal, rheumatologic, and pulmonary disorders, should not receive live-virus vaccines. For patients receiving long-term immunosuppressive therapy, clinicians should discuss the risks and benefits of vaccination and/or revaccination with an infectious disease specialist.

Patients with HIV infection should generally receive inactivated vaccines (eg, diphtheria-tetanus-acellular pertussis [Tdap], polio [IPV], Haemophilus influenzae type b conjugate [Hib]) according to routine recommendations. Despite the general caution against giving a live-virus vaccine, patients who have CD4 counts ≥ 200/mcL (ie, are not severely immunocompromised) can be given certain live-virus vaccines, including MMR. Patients with HIV infection who have not received a conjugate pneumococcal vaccine or whose previous vaccination history is unknown should be given PCV15, PCV20, or PCV21; if PCV15 is administered, follow with PPSV23 ≥ 8 weeks after the PCV15 dose. type b conjugate [Hib]) according to routine recommendations. Despite the general caution against giving a live-virus vaccine, patients who have CD4 counts ≥ 200/mcL (ie, are not severely immunocompromised) can be given certain live-virus vaccines, including MMR. Patients with HIV infection who have not received a conjugate pneumococcal vaccine or whose previous vaccination history is unknown should be given PCV15, PCV20, or PCV21; if PCV15 is administered, follow with PPSV23 ≥ 8 weeks after the PCV15 dose.

Patients with asplenia are predisposed to overwhelming bacteremic infection, primarily because of encapsulated organisms such as Streptococcus pneumoniae, Neisseria meningitidis, or H. influenzae type b. Adults with asplenia should be given the following vaccines (before splenectomy if possible):

• Hib conjugate vaccine: Patients are given a single dose and no booster.

• Meningococcal conjugate vaccine (MenACWY): Patients are given 2 doses at least 8 weeks apart and a booster every 5 years.

• Meningococcal B vaccine (MenB): Patients are given a 2-dose series of MenB-4C ≥ 1 month apart or a 3-dose series of MenB-FHbp at 0, 1 to 2, and 6 months (if dose 2 was administered at least 6 months after dose 1, dose 3 is not needed; if dose 3 was administered < 4 months after dose 2, a fourth dose should be administered at least 4 months after dose 3). MenB-4C and MenB-FHbp are not interchangeable (use same product for all doses in series). Patients are given 1 dose of MenB booster 1 year after the primary series and are revaccinated every 2 to 3 years if risk remains.

• Pneumococcal conjugate (PCV15, PCV20, and PCV21) and pneumococcal polysaccharide (PPSV23) vaccines: Patients are given 1 dose of either PCV20 or PCV21 or 1 dose of PCV15 followed by 1 dose of PPSV23 1 year apart if they have not previously received a pneumococcal conjugate vaccine or if their vaccination history is unknown. A shorter minimum interval of 8 weeks between PCV15 and PPSV23 can be considered for adults with an immunocompromising condition (including congenital or acquired asplenia), cochlear implant, or cerebrospinal fluid (CSF) leak.

Additional doses may be administered based on clinical judgment.

Before solid organ transplantation, patients should receive all appropriate vaccines. Patients who are status post-allogeneic or -autogeneic hematopoietic stem cell transplantation should be considered unimmunized and should receive repeat doses of all appropriate vaccines. Clinical posttransplant care of these patients is complex, and vaccination decisions for these patients should involve consultation with the patient's hematologist-oncologist and an infectious disease specialist. Live vaccines are generally contraindicated for up to 2 years posttransplant (3).

Live-microbial vaccines should not be administered simultaneously with blood or plasma transfusions or immune globulin; these products can interfere with the development of desired antibodies. Ideally, live-microbial vaccines should be administered 2 weeks before or 6 to 12 weeks after the immune globulins (4).

Live-microbial vaccines include the following:

• Bacille Calmette-Guérin (BCG) (for Bacille Calmette-Guérin (BCG) (fortuberculosis)

• Cholera vaccine

• Ebola vaccine

• Influenza virus vaccine (LAIV)

• Malaria vaccines

• Measles/mumps/rubella (MMR) vaccines

• Measles virus/mumps virus/rubella virus/varicella virus (MMRV) vaccine

• Polio (oral preparation only; no longer licensed or available in the United States)

• Rotavirus vaccine

• Smallpox and monkeypox vaccine, nonreplicating

• Oral typhoid vaccine

• Varicella virus vaccine

• Zoster vaccine (only the recombinant vaccine is available in the United States)

• Yellow fever vaccine

1. 1. Abara WE, Gee J, Marquez P, et al. Reports of Guillain-Barré Syndrome After COVID-19 Vaccination in the United States [published correction appears in JAMA Netw Open. 2023 May 1;6(5):e2315740. doi: 10.1001/jamanetworkopen.2023.15740.]. JAMA Netw Open. 2023;6(2):e2253845. Published 2023 Feb 1. doi:10.1001/jamanetworkopen.2022.53845

2. 2. Bishara H, Arbel A, Barnett-Griness O, et al. Association Between Guillain-Barré Syndrome and COVID-19 Infection and Vaccination: A Population-Based Nested Case-Control Study. Neurology. 2023;101(20):e2035-e2042. doi:10.1212/WNL.0000000000207900

3. 3. Silva-Pinto A, Abreu I, Martins A, Bastos J, Araújo J, Pinto R. Vaccination After Haematopoietic Stem Cell Transplant: A Review of the Literature and Proposed Vaccination Protocol. Vaccines (Basel). 2024;12(12):1449. Published 2024 Dec 23. doi:10.3390/vaccines12121449

4. 4. National Center for Immunization and Respiratory Diseases. General recommendations on immunization --- recommendations of the Advisory Committee on Immunization Practices (ACIP) [published correction appears in MMWR Recomm Rep. 2011 Jul 29;60:993]. MMWR Recomm Rep. 2011;60(2):1-64