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The gift of vaccines
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Vaccines against 25 infectious diseases have been licensed by the FDA and are available in the United States. Vaccination against 16 diseases is recommended routinely in the CDC’s childhood immunization schedule, and another nine vaccines are recommended for people at specific risk or at certain ages (see Figure).
The effect of vaccines on disease reduction is difficult to exaggerate. The most dramatic example of a successful immunization program is the elimination of smallpox — not only from the United States but globally (the last naturally occurring case of smallpox was in October 1977). Now, the global eradication of poliomyelitis is anticipated eagerly. Members of the Polio Global Eradication Initiative project the last case of naturally occurring polio will occur in 2014. What greater gift could be passed on to future generations than the elimination of both poliomyelitis and smallpox?
Great success in disease prevention
Since the start of the second half of the 20th century, rates of vaccine-preventable diseases have plummeted. The total number of reported cases of invasive Haemophilus influenzae type b disease in 2012 among children younger than 5 years was 30. The number of reported cases of invasive Streptococcus pneumoniae in 2012 among young children was 835. Other vaccine-preventable diseases reported in 2012 are at or near historic lows, including only nine cases of rubella, 147 cases of measles and 404 cases of mumps.
H. Cody Meissner
The number of reported cases of other vaccine-preventable diseases continues to fall: hepatitis B (2,100 reported cases in 2012); hepatitis A (1,130 cases reported in 2012); varicella (9,800 reported cases in 2012); meningococcal disease (161 cases caused by the four vaccine serogroups in 2012); and tetanus (37 cases in 2012). Pertussis remains a lonely reminder of a disease for which an improved vaccine is needed.
Recently licensed vaccines are showing early results that are equally compelling. Since introduction of the oral rotavirus vaccine in 2006, there has been a reduction of more than 90% in hospitalizations and ED visits attributable to rotavirus gastroenteritis.
Perhaps even more spectacular are the preliminary estimates of disease reduction attributable to oncogenic HPV types. The HPV vaccine first was recommended for use as part of the routine immunization schedule in 2006 for females aged 11 or 12 years. By 2009, a 24% reduction in the prevalence of anogenital warts in 15- to 19-year-old women was reported, whereas the prevalence of warts in other (non-vaccinated) female age groups increased.
A CDC report earlier this year describes a 56% reduction in the prevalence of vaccine HPV strains from cervicovaginal swab samples obtained from females aged 14 to 19 years, during a period when vaccine uptake has been disturbingly limited. Among other age groups, no change in prevalence of vaccine types was found between the prevaccine era and 2007 to 2010, demonstrating clearly the benefit of early immunization. Although more than 1 decade will be required to demonstrate a reduction in the number of cases of cancer, many of the 19,000 cancers occurring each year attributable to HPV vaccine types 16 and 18 in both men and women will be reduced dramatically.
A bright future for vaccines
What vaccines are possible in the future? A glycoconjugate vaccine for prevention of neonatal group B streptococcal (GBS) disease is in development. Effective maternal immunization during gestation could prevent most cases of early- and late-onset GBS disease. A respiratory syncytial virus vaccine for immunization of pregnant women holds the promise of protecting all infants against severe RSV infection, not just those infants at increased risk for severe disease. By boosting maternal antibody followed by transplacental transfer, immunity can be provided for all infants during the first few months of life.
A phase 1/2 trial of a novel multivalent outer surface protein A vaccine for Lyme disease has been reported. This vaccine would protect against Borrelia burgdorferi strains found in the United States, as well as those in Europe, and would be recommended for people at increased risk because of activity in a tick habitat.
Vector control has had little effect on the spread of dengue and no safe and effective dengue vaccine is available to control the current pandemic. Several vaccine candidates for dengue virus are at various stages of development.
One-third of the world’s population is infected with Mycobacterium tuberculosis. Most infected people have latent disease and are asymptomatic, but 5% to 10% of those with latent TB will develop active TB. Worldwide, more than 8 million new cases of active TB occur each year, and with the emergence of multidrug-resistant TB, the need for an improved vaccine is great. Vaccines under consideration may either replace bacillus Calmette-Guérin (BCG) completely or increase the efficacy of BCG. Progress in vaccine development has been hampered by lack of understanding of the immune correlate of vaccine-induced immunity, but new developments are providing insight into the design of future TB vaccines.
Other examples of vaccines in development include an inactivated enterovirus 71 vaccine, a norovirus vaccine (the leading cause of gastroenteritis in industrialized countries) and a subunit vaccine against herpes simplex type 1 and 2.
The control of vaccine-preventable diseases in the United States has been nothing less than spectacular. All too often the accomplishments of disease eradication are forgotten, but vaccines remain one of the most promising tools for control of existing — as well as emerging infectious diseases. It is not a coincidence that in countries with high immunization rates, infectious diseases are often at historically low rates.
The success of an immunization program depends on three factors. First, the public must understand the worth of vaccines and be confident in their safety. Second, manufacturers must be willing to accept the enormous financial risk associated with vaccine development. Expectation of sufficient profit from manufacture of a safe and effective vaccine is essential to encourage participation by the pharmaceutical industry. Finally, public health professionals must ensure that public health needs regarding vaccines are identified and appropriately addressed.
The effect of changes in the earth’s global climate on infectious diseases is difficult to predict, but, unquestionably, new diseases will continue to emerge. Vaccines will be the basis for countering those challenges.
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Disclosure: Meissner reports no relevant financial disclosures.