“For the Herd’s Sake, Vaccinate” was the title of an editorial published online Dec. 27 in The New York Times by Steven L. Weinreb, MD, an internist certified in oncology and hematology with chronic lymphocytic leukemia.
He recently had received an allogeneic stem cell transplant. Dr. Weinreb was concerned that he might now be susceptible to vaccine-preventable diseases to which he had previously been immune and that his protection from those diseases relied on a high level of community immunity. This level of community protection, or what is more commonly called “herd immunity,” acts as a barrier to transmission, thus protecting people such as Dr. Weinreb by reducing the potential for exposure to infectious agents.
Walter A. Orenstein
Most vaccine-preventable diseases are transmitted from person-to-person in what is known as a chain of transmission — an infectious person coming in contact with a susceptible person and transmitting the infectious agent. After an incubation period, the newly infected person becomes contagious and transmits infection to others who are susceptible, maintaining the transmission chain. When the infectious person comes in contact with an immune individual, transmission does not occur and the chain is not extended. Infectious people do not have infinite contacts. The degree of contagiousness varies by disease and is often quantified by mathematical modelers as R0, the basic reproduction number.
R0 is the average number of secondary cases that would occur when an infectious person comes in contact with a population that is 100% susceptible. For example, if R0 is four, then the average transmitting case is capable of infecting four people. If the population immunity level is 75%, then three of those four will be immune, leading to only one secondary case — in other words a constant level of cases. If the immunity level goes above 75%, the average transmitter will give rise to less than one case and transmission will die out. The level at which transmission is one case is known as the herd immunity threshold and is calculated as (R0-1)/R0. Thus, communities can be protected without having all residents immune. People indirectly protected include children too young for effective vaccination; those with compromised immune systems who cannot make an effective response to vaccines; people with contraindications to vaccination (eg, allergies to vaccine components, immunosuppression, etc); people who did not make a protective immune response to vaccines they received or who lost immunity with increasing time since vaccination; and people who are unvaccinated for whatever reason (eg, refusal, lack of access, etc).
According to a recent review by Fine and Mulholland, the R0 for rubella is six or seven, whereas the R0 for measles is 12 to 18. Hence, a person with measles is roughly twice as contagious as a person with rubella, and eliminating measles requires a higher herd immunity threshold than rubella. In fact, measles is associated with one of the highest R0 values of the vaccine-preventable diseases and is the disease we often see first when our community immunity level falls below the threshold.
We often think of our community as the domestic United States. But that is much too parochial a view. Most vaccine-preventable diseases affect children around the globe, and for a number of these infectious agents, children in developing countries are disproportionately affected. As recently as 2000, an estimated 733,000 children died of measles globally. Even in 2008, that number was still unacceptably high, 164,000. Further, measles is frequently imported into the United States, either through susceptible US residents traveling to countries where it is endemic or epidemic and then brought back to the United States or from travelers from those countries who come to the United States (three to eight importations a year from 2001 to 2010). These introductions can lead to measles outbreaks.
Vaccine-preventable disease outbreaks
The largest outbreak of measles in the United States in 2011, 22 cases, was reported from Minnesota and was the result of an importation of virus from Kenya. Another outbreak was started in Indiana by a 24-year-old US resident infected in Indonesia, leading to 14 cases. Investigation of the Indiana outbreak identified 780 people exposed to someone with measles who had to be followed to determine if they became ill. Costs of controlling a measles outbreak can be considerable. An investigation of an outbreak in Utah, resulting from exposure in Europe, led to identification of 49 suspected or confirmed cases, quarantining of 184 people, and notification of 12,000 contacts and substantial emergency vaccination efforts at a cost of nearly $300,000.
Measles is not the only vaccine-preventable disease of concern. An estimated 53,800 chronic hepatitis B cases were imported into the United States annually from 2004 to 2008 from immigrants. Polioviruses have been detected, silently circulating in the United States as recently as 2005. In 2003, a US citizen died in Pennsylvania of diphtheria, which was acquired in Haiti.
Regarding vaccine-preventable diseases, the best defense for the United States is a good offense in reducing, eliminating or eradicating these diseases in other countries, which are reservoirs for the infectious agents. The ultimate benefit to the world, including the United States, was the global effort to eradicate smallpox. The last naturally occurring case of smallpox was detected in Somalia in 1977. Successful smallpox eradication was the result of a global coordinated effort to rid the world permanently of a disease that killed approximately 30% of people affected. As a result, every child in the United States and globally is now protected against smallpox, not because they are vaccinated but because they are not exposed.
Since 1988, when there were an estimated 350,000 people paralyzed by polio, there has been a global effort to eradicate the disease (see www.polioeradication.org). Cases have been reduced by more than 99%, and as of January 2011, only three countries have never interrupted transmission within their country, compared with 125 when the effort began. But now is not the time to let up, or polio will resurge. Resources and political commitment must be intensified or the few remaining countries with the virus will re-infect many others. Measles is targeted for elimination in five of the six WHO regions. But we have seen let ups in a number of areas with a return of measles epidemics.
Role of the pediatrician
What can pediatricians do to help reduce the burden of vaccine-preventable diseases around the world? First, they can assure that their patients are fully immunized and check when those patients travel that they receive all vaccines recommended for international travel. Information on international travel vaccine recommendations can be found at: wwwnc.cdc.gov/travel/page/vaccinations.htm. This is particularly important for measles: Children as young as 6 months of age may be vaccinated because measles in young infants tends to be more severe than at older ages.
Second, make polio eradication a reality. Pediatricians can work with rotary clubs in their communities to volunteer and advocate. More information on polio eradication can be found at www.polioeradication.org.
Third, advocate with government officials about the importance of US technical and funding support to global immunization programs. More information on what pediatricians can do may be found at www2.aap.org/immunization/about/globalpartnerships.html.
In most of the world, national governments provide leadership to their immunization programs. Through the Expanded Programme on Immunization (EPI) (www.who.int/immunization/en), WHO provides technical leadership and guidance, and UNICEF (www.unicef.org/immunization/index.html) plays a major role in immunization program implementation. The GAVI alliance (www.gavialliance.org), along with multiple bilateral donors, helps finance immunization programs, particularly with a focus on making newer vaccines, such as pneumococcal conjugate vaccines, rotavirus vaccines and many others, available to some of the poorest countries in the world.
Supporting global immunization is a win-win. First, it is a win for children in the developing world, many of whom are suffering from and dying of vaccine-preventable diseases needlessly. Second, it is a win for our own children, decreasing the chances they will ever be exposed to a vaccine-preventable disease.
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- Weinreb SL. For the herd’s sake, vaccinate. The New York Times. Dec. 28, 2011:A23 (New York edition).
- Dr. Orenstein reports no relevant financial disclosures.