Kristen A. Feemster, M.D.,M.P.H., is an assistant professor of pediatrics at Children's Hospital of Philadelphia, where she sees patients and conducts research on infectious disease epidemiology and vaccine policy.
By Kristen A. Feemster
An old disease is back. Cases of pertussis, or whooping cough, have more than tripled in the past five years, with 2012 on track to be the most severe in over a half-century. More than 37,000 cases have been reported so far – 37 times the number in 1976, which was the lowest since the introduction of vaccines dramatically reduced prevalence of the disease.
Philadelphia is one of the communities that have been hit particularly hard. At my home institution in West Philadelphia, we have had 346 confirmed cases diagnosed since July 2011, compared to the usual average of one to two a month. Anyone can be infected but infants are most likely to develop severe illness. Two out of three infected infants will need to be hospitalized, and 1 in 100 infants with whooping cough will die from it – this is a disease to take seriously.
Before pertussis vaccine was introduced in the 1940’s, there were an average of 115,000-270,000 cases per year in the United States, and 5,000-10,000 deaths. Disease rates have dropped significantly since then. So what is happening now? The alarming increase in pertussis cases is the result of a combination of factors: the challenge of controlling a highly contagious – and often under-recognized – disease, and the reality of an imperfect vaccine in an era of increasing public distrust of vaccines.
Pertussis is a highly contagious disease. It is spread from one person to another through respiratory droplets, such as from coughing or sneezing, and some 90 percent of susceptible people living with an infected person will get sick. Because of the ease with which the bacterium, Bordetella pertussis, spreads, almost everyone in a community needs to be immune in order to stop transmission. This requires high vaccination rates, especially in places where there are individuals (mainly young infants) who are not able to be fully vaccinated. Additionally, pertussis is often under-diagnosed, because the symptoms can sometimes be difficult for even physicians to recognize.
The infection starts with a mild cough and runny nose that is difficult to distinguish from a common cold. After one to two weeks, a severe cough develops. Classically, this is marked by episodes of multiple, rapid coughs without any break that may be so severe, the lungs run out of air. The sound of pertussis from these episodes is unmistakable whooping sound. But many infected people, especially older children, adolescents and adults, do not develop classic symptoms. These groups may have a mild but persistent cough that could be diagnosed as other causes of chronic cough, such as asthma. (To hear the various sounds of pertussis, click here.) And because infected people are contagious for up to three weeks after developing symptoms, those with unrecognized disease often unknowingly spread it to others. When it is difficult to identify cases, of course, it is also difficult to control transmission – so preventing infection through vaccination must be a key part of our strategy.
The vaccine is effective, but imperfect. A flurry of articles in newspapers and the medical literature have suggested various reasons for the pertussis epidemic, which has hit some western states particularly hard. One issue that has recently garnered widespread attention is waning immunity. An article published in the New England Journal of Medicine in September described decreasing protection from pertussis among children even after getting all of the recommended doses of pertussis vaccine. This has prompted some parents to question why their children should receive the vaccine at all. Waning protection after immunization does not mean that this is a bad vaccine. The vaccine does work – it prevents disease – but protection does not last as long as had been expected.
In the 1990s, the U.S. switched from what was known as the “whole cell” pertussis vaccine to one with fewer side effects, referred to as the “acellular” pertussis vaccine. The older, whole cell vaccine had more antigens, or parts of the bacteria, for the human body to build immunity against. This produced a stronger immune response, but with more side effects. The new vaccine traded some immunity for fewer side effects. This “acellular” version effectively prevents disease, but immunity wanes over time, which means that we can become susceptible to infection again. Waning immunity also occurs after having natural pertussis infection. In both cases, what remains is often called “partial protection,” meaning that a person can be infected but may not get as sick. This is why older children and adults are more likely to have milder symptoms. Unfortunately, in this scenario, we can still transmit disease. While a better vaccine is warranted, we need to optimize our use of what is currently available. Recommendations were expanded recently to include booster doses for adolescents and now all adults.
Meanwhile, there is some public distrust of vaccines. News about waning protection after childhood pertussis vaccination may add to already heightened concerns about vaccine safety. Parents' refusal to vaccinate their children – acceptable reasons to opt out of mandatory immunization varies from state to state – has resulted in increased risks of outbreaks for vaccine-preventable diseases in certain communities. In fact, some of the local pertussis outbreaks have provided evidence for what happens when too many members of a community skip vaccination. Any additional challenges to the public’s trust in pertussis vaccines will further threaten our ability to effectively control this epidemic.
While we do not have a perfect vaccine, adhering to current vaccine policies is our best strategy to prevent the spread of whooping cough and protect those who are most vulnerable. The most severe cases of pertussis occur in young infants who have not yet received the full vaccine series and, therefore, do not have immunity. They are the most likely to be hospitalized and die; in fact, we have seen deaths from pertussis in the Philadelphia regionAs a pediatrician, I find it difficult to see a child die from a disease that is preventable. We need to do what we can to minimize their risk. For young infants, the most likely source of transmission is caregivers – studies have shown that 75 percent of infant cases were passed on by a household contact or caregiver, most often the mother (33 percent) or father (16 percent), who may not know that they are infected. For that reason, the federal Advisory Committee on Immunization Practices recommends “cocooning”: immunizing everyone who has close contact with young infants including parents, grandparents, siblings and out-of-home caregivers.
The advisory committee also recently recently recommended that all pregnant women receive Tdap (combination tetanus, diphtheria and pertussis vaccines) during each pregnancy. When pregnant women are vaccinated, the protection is two-fold. First, they develop antibodies to pertussis, decreasing their chance of getting pertussis and giving it to the baby. Second, they pass antibodies to the baby before birth, across the placenta, and after birth, through breast milk, protecting the infant before immunization.
An old disease is back in a very new world. We have tools that are effective, but not perfect, to prevent what once was and could again be a source of significant illness and death, especially for the youngest among us. As we work to develop even better tools, it is imperative that we continue to use those already at our disposal to protect ourselves, our families and our neighbors.
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