Most vaccines are injected by subcutaneous, intramuscular, or intradermal routes. Oral vaccines are available for only a few diseases, but they have some distinct advantages. An oral dose of a vaccine can stimulate protection (IgA) on the mucous mem brane of the portal of entry. Oral vaccines are also easier to give, more readily accepted, and well tolerated. Other methods that show promise are an intranasal vaccine delivered into the nose by aerosol or drops (FluMist for influenza, for example) and skin patches.
Some vaccines are made more effective by adding an adjuvant. An adjuvant is any compound that enhances immunogenicity and prolongs antigen retention at the injection site. The adjuvant precipitates the antigen and holds it in the tissues so that it will be released gradually. Its gradual release presumably facilitates con tact with antigen-presenting cells and lymphocytes. Common adjuvants are alum (aluminum hydroxide salts), Freund’s complete adjuvant (emulsion of mineral oil, water, and extracts of mycobacteria), and beeswax.
Vaccines must go through rigorous trials in experimental animals and human volunteers before they are licensed for general use. Even after they have been approved, like all therapeutic products, they are not without complications. The most common of these are local reactions at the injection site, fever, allergies, and other adverse reactions. Relatively rare reactions (about one case out of 300,000 vaccinations) are panencephalitis (from measles vaccine), back-mutation to a virulent strain (from polio vaccine), disease due to contamination with dangerous viruses or chemicals, and neurological effects of unknown cause (from pertussis and swine flu vaccines). Some patients experience allergic reactions to the medium (eggs or tissue culture) rather than to vaccine antigens.
When known or suspected adverse effects have been detected, vaccines are altered or withdrawn. For example, the whole-cell pertussis vaccine was replaced by the acellular capsule (aP) form when it was associated with adverse neurological effects, and the Lyme disease vaccine was withdrawn when recipients developed arthritis and other side effects. Vaccine companies have also phased out certain preservatives, such as thimerosal, that could cause allergies and other medical problems in some patients.
Professionals involved in giving vaccinations must understand their inherent risks but also realize that the risks from the infectious disease always outweigh the chance of an adverse vaccine reaction. The greatest caution must be exercised in giving live vaccines to immunocompromised or pregnant patients, the latter because of possible risk to the fetus. Unfortunately, vaccine hesitancy is too often linked to political affiliation—where vaccines may be considered government overreach, or simply support of the “other guy”— along with the residue of toxic social media posts, where the most baseless rumors may have an unending life of their own. The con sequences of not vaccinating are being felt in some populations. Beyond the thousands of (unvaccinated) deaths due to COVID-19, recent outbreaks of measles, mumps, meningitis, and pertussis have been tied to inadequate vaccination. The reality is that these dis eases are still around, and they pose a greater risk for young children than the possibility of a vaccine reaction. In fact, 1% of babies develop a severe brain condition from measles virus, and mumps, meningitis, and pertussis all have their own serious complications. Any reduction in vaccinations will lead to less herd immunity and will create pockets where children lack protection. Now more than ever before, health care workers have a responsibility to educate and counsel patients about the importance of routine immunizations for both individuals and the community.
