Vaccines are medications that boost our ability to fight off certain diseases. Many of the vaccine-preventable diseases are highly contagious and even fatal in non-immunized individuals (Table 1). Prior to the development of vaccines, these diseases disabled or killed millions of people. Many people living in developed countries today do not appreciate the value of immunizations because the successful use of vaccines has almost eradicated many of these diseases. These diseases are still dangerous and can kill people who are not adequately protected from them.

Immunization is the act of receiving a vaccine. Immunity is the ability of the body to recognize specific infecting organisms as foreign and thereby protect against them.

Immunity (protection) can occur one of two ways:

• Active immunity
  • The first way to become immune is by actually getting the natural disease. For many organisms, this confers immunity for life. When the person is exposed again to the organism, the immune system quickly reestablishes protection.
  • The second way to become immune is through the use of a vaccine. The vaccine interacts with the immune system and creates the same protection as if the person had the natural infections. This is done without being exposed to the risks involved with getting the natural infection.
• Passive immunity
  • Newborn babies receive immunity from their mothers. This only lasts for a period of months, and this is why newborns need to start receiving immunizations shortly after birth.
  • For some diseases, there are shots that contain antibodies against some diseases (immune globulin). Examples of diseases for which we have immune globulin include rabies and hepatitis.

There are two major categories of vaccines.

• The first category of vaccine is made from live viruses that have been "attenuated" or weakened so that they do not cause the disease (Table 2). Usually, any symptoms caused by the vaccine is milder than the natural disease. The attenuated viruses elicit a strong immune response as the virus is very close to the virus that causes the disease.
• The second category of vaccine, inactivated vaccine, is produced by growing the bacterium or virus in culture and then inactivating it (killing it) by using heat or chemicals (Table 3). These vaccines cannot cause the disease, but allow the body to develop immunity. While these vaccines are safer, they do not produce protection as good as that from the live vaccines.

Simultaneous administration (vaccines given at the same visit but not in the same shot) of most commonly used vaccines does not decrease the response to the vaccines or increase the risk for adverse reactions. The simultaneous administration of vaccines was instituted to increase compliance with recommended immunization schedules. If people have to come back many times to get additional shots, there is an increased chance that they will not get all recommended vaccinations. In children, there are now a few combination shots that contain multiple vaccines in a single shot. Most of these are approved for use in adults, except Tdap (a three-vaccine combination) and one containing measles/mumps/rubella (MMR). There is an ongoing controversy in the public media about giving "too many" vaccines at one time to little children. Physicians, however, do not believe that children are at risk from "too many" vaccinations given at one time.

There is no such thing as a risk-free vaccine. However, the health risk of not being vaccinated is real and is clearly greater than that of being vaccinated. Most side effects from vaccinations are mild and limited to local reactions at the injection site and/or a mild fever. Unfortunately, there are rare serious and even fatal side effects related to vaccines. While these events are sad, not taking the vaccine could also result in death or disability.

In December 2017, the American College of Allergy, Asthma, and Immunology published an updated guideline and recommended that patients with egg allergy receive the influenza vaccine. Even though the vaccine is currently manufactured in eggs, there is only minimal egg protein in the vaccine. There is no increased risk of reaction in patients with egg allergy.

With this new recommendation to utilize the routine influenza vaccine in patients with egg allergy, there is no need to utilize the egg-free vaccine that was released in 2013. Therefore, although the recombinant hemagglutinin influenza vaccine (RIV) is not made using eggs, it is no longer needed.

Most reactions to vaccines are mild and self-limited. These are usually limited pain, swelling, and redness at the site of the vaccination. These occur in up to 80% of individuals and start within hours of the vaccination. Some people can get more generalized symptoms, including fever, muscle aches, headache, loss of appetite, and feeling generally tired. These systemic (generalized) reactions are seen more commonly with live attenuated vaccines and usually occur seven to 21 days after the vaccine was given. The worst (and very uncommon) reaction is anaphylaxis (a severe allergic reaction). These reactions usually occur shortly after the vaccine is given and can be life-threatening. Fortunately, these reactions only occur two times for every million doses of vaccine given.

There are two types of contraindications (reasons not to give a vaccine): permanent and temporary.

• The following are permanent contraindications to vaccination:
  • severe allergic reaction to a vaccine component (animal proteins [eggs], antibiotic, stabilizer, or preservative) or following a previous dose of the vaccine;
  • encephalopathy within seven days of a pertussis vaccination (not from another identifiable cause). This reaction is very rare since the introduction of acellular pertussis vaccine.
• The following are precautions/temporary contraindications to vaccination:
  • Pregnancy: Although the risk of vaccination during pregnancy is mostly theoretical, caution is advised. Therefore, women who are known to be pregnant should not receive any of the live vaccines (Table 2). Inactivated vaccines are considered generally safe during pregnancy and should be used when indicated (Table 3). See the guide from the U.S. CDC for a complete listing of approved vaccines during pregnancy.
  • Immunosuppression: People with active cancer, leukemia, or lymphoma (or people taking high doses of steroids) should not receive live vaccines but can receive inactivated vaccines.
  • Human immunodeficiency virus (HIV): Vaccination depends on the severity of the illness. In asymptomatic (without symptoms) individuals, many vaccines are considered safe. In general, the inactivated vaccines are safe for both symptomatic and asymptomatic individuals infected with HIV.
  • Moderate to severe illness: If someone is ill with more than a simple cold, earache, diarrhea, or other minor illness, vaccination should be postponed until the illness is over.

Women who are pregnant should not receive the MMR, oral typhoid yellow fever, varicella, or zoster vaccines. These vaccines are made from live attenuated viruses and potentially could cause a problem. Pregnant women may receive tetanus and influenza vaccines as needed. It is safe to receive hepatitis A & B, meningococcal, and pneumococcal vaccines.

Vaccination should not be postponed for any of the following reasons:

• Mild illness: Low-grade fever, colds, upper respiratory-tract infections, and mild diarrhea are not reasons to put off vaccination.
• Antibiotics: The current administration of antibiotics is not a reason to put off vaccination.
• Household contacts of pregnant women or immunosuppressed patients: Living in a house with a pregnant woman or an immunosuppressed patient is not a reason to put off vaccination. Two exceptions are the live attenuated nasal influenza vaccine (which should no loner be used for anyone due to its lack of efficacy) and smallpox vaccine.
• Breastfeeding: Breastfeeding is not a reason for either the mother or baby to put off vaccination.
• Preterm birth: Preterm birth is not a reason to put off vaccination.
• Generalized allergies: Children with allergies, but no history of reactions to vaccine components, should receive vaccines as recommended.
• Family history: Having a family member who had an adverse reaction to a vaccine is not a reason to put off vaccination.

The side effects of most vaccines are mild and go away with in a few days. Common side effects of many vaccines include soreness at the injection site, low-grade fever, fatigue, headache, and muscle aches. However, some vaccines can have serious or even life-threatening reactions. Thousands of lives are saved for every serious side effect that is caused. The specific side effects of specific vaccines are found at https://www.cdc.gov/vaccines/vac-gen/side-effects.htm.

Vaccine-preventable diseases are indeed infrequent in the United States because of the success of the vaccine program. However, if people stop receiving vaccines, these diseases will come back rapidly. This occurred with measles in the early 1990s and resulted in many deaths.

If the reaction is a mild reaction with just some injection-site tenderness, low-grade fever, fatigue and headache, then simply taking a dose or two of acetaminophen (Tylenol) may be helpful. The use of ibuprofen or other nonsteroidal anti-inflammatory drugs has been discouraged because of the concern that these drugs might decrease the immune response after the vaccine (make the vaccine less effective). Studies are still ongoing at this time. It is best to check with the physician who ordered the vaccine if you are concerned about the symptoms or you want to take something for the symptoms.

If you think you are having a serious side effect, consult your physician immediately. If a person is seriously injured by a vaccine, there is available compensation through the National Childhood Vaccine Injury Act.

In 1986, the United States government set up the National Childhood Vaccine Injury Act. This act provides a "no fault" compensation mechanism for people injured by vaccines.

Health care workers are treated a little differently than other adults for two reasons. First, a health care worker is more likely to be exposed to certain risks of infection (such as hepatitis B) than the normal population. Second, if a health care worker becomes infected, they may transmit those infections to their patients (chickenpox, pertussis).

Special recommendations

• Tetanus/diphtheria/pertussis (Td/Tdap)
  • It is recommended that any health care worker who may have patient contact receive a Tdap shot if they have not received one as an adolescent (as long as it has been two years since their last Td shot). This helps prevent the spread of pertussis.
• Hepatitis B
  • Health care workers who have not been vaccinated should receive the three-dose series and obtain anti-hepatitis B serology testing one to two months after their third dose.
• Measles/mumps/rubella (MMR)
  • There must be either documented evidence of all three diseases (measles, mumps, and rubella) or serologic evidence of immunity (determined by a blood test) in anyone born after 1957. If there is no serologic evidence of immunity, the health care worker should receive two doses of MMR separated by 28 days or more.
• Varicella
  • All health care workers must have a history of varicella disease (chickenpox), prior vaccination, or serologic evidence of immunity. If not, the worker should receive two doses of vaccine 28 days apart.
• Influenza
  • Health care workers should receive one dose of the flu shot annually.

There are a number of vaccines that are not routinely given in the U.S. that are recommended for foreign travel. Which vaccines a person needs completely depends on the country to which he or she is traveling. The CDC has a web site that is constantly updated with recommendations for vaccines (https://wwwnc.cdc.gov/travel/). People can learn about recommended vaccines by simply navigating to the country they plan to visit. These vaccines can then be obtained from many local health departments or travel clinics.

Travelers going to sub-Saharan Africa and tropical South America are required by International Health Regulations to have yellow fever vaccination. All other vaccines are simply recommended to protect the traveler. Common vaccines given for foreign travel include hepatitis A vaccine, hepatitis B vaccine, and typhoid vaccine. U.S. citizens who have not received their recommended (routine) vaccinations (Tdap, MMR, polio, etc.) should make sure that they get all routinely recommended vaccines before traveling. Many of these diseases are still very common in other parts of the world. For instance, people going to Mecca on the Haj are required to be vaccinated against meningitis.

Until 2017, there was an influenza vaccine that could be given as a spray in the nose. This vaccine is no longer available because it simply did not work as well as the shot.

1. Centers for Disease Control and Prevention web site for vaccines and immunizations at https://www.cdc.gov/vaccines/index.html. This is updated annually in the fall of the year.
2. Epidemiology and Prevention of Vaccine-Preventable Diseases: The Pink Book: Course Textbook Updated 13th Edition (May 2015) at https://www.cdc.gov/vaccines/pubs/pinkbook/index.html
3. Recommendations on immunization for health care workers at http://www.immunize.org/catg.d/p2017.pdf
4. http://www.immunize.org/
5. http://www.immunizationed.org/
6. Traveler information: https://wwwnc.cdc.gov/travel