The yellow fever vaccine that has been in use for more than 80 years is a live, attenuated vaccine is a type of vaccine that uses a live, but weakened form of the virus or bacteria that causes a disease. This weakened strain of the pathogen is still able to stimulate an immune response in the body, but it is not strong enough to cause illness. Other examples of live, attenuated vaccines include the measles, mumps, rubella (MMR) vaccine, oral typhoid and varicella (chickenpox).
Interestingly, the attenuated, live yellow fever virus was developed through a chance mutation during animal experimentation. The process of cultivating the attenuated virus for use in the vaccine has led to additional mutations that have further weakened the attenuated virus.
Is it safe to get multiple live vaccines at the same time?
Yes, it is generally safe to receive multiple live vaccines at the same time. The Centers for Disease Control and Prevention (CDC) recommends simultaneous administration of live vaccines as it can be more convenient for the person receiving the vaccines and may also increase the chances that the person will be fully protected. However, it is always best to consult with your healthcare provider to determine the best schedule for your vaccines.
Are live vaccines safe for everyone?
The yellow fever vaccine is a live attenuated vaccine, and it may not be safe for people with weakened immune systems. This includes people with HIV or AIDS, thymus disorder, cancer or other conditions that affect the immune system, and people taking immunosuppressive medications. Adults over 60 years old, babies between 6 and 8 months old, pregnant and nursing mothers are also included in the "may not be safe" group. The yellow fever vaccine should not be administered to children under 6 months old.
If you are in the "may not be safe" group, you have three options:
Get the vaccine anyway
Avoid travel to a country with endemic yellow fever
Get a yellow fever waiver for entry into countries that require proof of yellow fever vaccination for entry
Why is there a waiting period between live vaccines?
You normally have to wait 28 days before receiving a live vaccine after receiving a different live vaccine. This is because live vaccines have the potential to interfere with each other, either by reducing the effectiveness of the second vaccine or by causing an increased risk of side effects.
For example, if a person receives a live attenuated measles vaccine and then receives a live attenuated yellow fever vaccine soon after, the measles vaccine may not be as effective because the yellow fever vaccine may interfere with the measles vaccine's ability to stimulate an immune response. Similarly, if a person receives two live vaccines too closely together, they may be at an increased risk of developing side effects from both vaccines.
Will they ever have an inactive yellow fever vaccine?
Currently, many different yellow fever vaccines are in development or testing stages [1]. These are mostly all inactivated (dead) virus vaccines. In response to the COVID-19 pandemic, new vaccine technology, called mRNA vaccine technology, was developed that may create new yellow fever options as well. The technology is advancing faster than the ability of researchers to test and produce vaccines.
Conclusions
It is important to talk to a travel health specialist about your specific vaccine schedule and timing. The U.S. CDC recommends specific guidelines that one should follow for each vaccine, to ensure that the best protection is received.
Sources
1. Montalvo Zurbia-Flores, G., Rollier, C. S., & Reyes-Sandoval, A. (2022). Re-thinking yellow fever vaccines: fighting old foes with new generation vaccines. Human Vaccines & Immunotherapeutics, 18(1), 1895644.