In a letter to the editor published in the New England Journal of Medicine, doctors have identified twelve cases of pertussis that do not respond to the pertussis vaccine. The samples were collected from children hospitalized in Philadelphia in 2011 and 2012. These are the first cases of vaccine-resistant pertussis identified in the United States.
Pertussis, also known as whooping cough, is caused by the bacterium Bordetella pertussis. The respiratory infection is spread when someone breathes in bacteria-contaminated air droplets from an infected person’s cough or sneeze. Transmission often occurs from older family members to infants or children.
The disease causes cold-like symptoms and severe coughing, often accompanied by a “whoop” sound. Patients can usually be treated successfully with antibiotics.
A huge spike in pertussis cases occurred in the United States this year with twice as many cases reported in 2012 compared with 2011. Twenty-one states experienced outbreaks above the national incidence levels, and an epidemic was declared in Washington in April, bringing the nationwide 2012 total to over 40,000.
To prevent pertussis, the CDC recommends that children receive five doses of the DTaP vaccine, which also includes protection against tetanus and diphtheria. Protection gained from pertussis vaccines does decrease over the years; this is known as “waning immunity.” For this reason, the CDC advises teens and adults to renew their protection with a TDaP vaccine.
These vaccines contain an antigen, called pertactin. Antigens are proteins found on the surface of pathogens. Each pathogen has antigens that are specific; the antigens on B. pertussis are different than the antigens on V. cholerae (cholera). Antigens are what trigger the immune system to produce antibodies, which fight the pathogen. Antigens and antibodies are shaped in such a way that only certain antibodies can fight certain antigens. Pertactin is an antigen typically found in B. pertussis. By adding the antigen to the vaccine, scientists were able to ignite an immune response in the human body and initiate protection against whooping cough.
That is, until now.
The vaccine will effectively protect people against B. pertussis infection if the body recognizes pertactin in B. pertussis. Therefore, if someone is infected with bacteria that do not express pertactin, the body cannot recognize the bacteria and kill it.
What the New England Journal of Medicine letter states is that doctors have found strains of B. pertussis that do not contain pertactin, therefore, our vaccines are not effective against them.
Pertactin-negative pertussis bacteria have already been identified in Japan, France and Finland. Researchers in France have begun more regular testing for these strains of bacteria in order to monitor bacterial mutations. In an upcoming article in Emerging Infectious Diseases, these same researchers report that the proportion of pertactin negative B. pertussis samples compared to the total number of samples collected each year has increased from two percent in 2005 to fourteen percent in 2012.
This new resistance to the pertussis vaccine may be why whooping cough cases have been increasing. Experts also believe that increased awareness about whooping cough has led to reporting of more cases.
While this news is significant, it is not entirely surprising. Several other pathogens have developed resistance to antibiotics. Both tuberculosis and gonorrhea, for example, have developed resistance to many of the drugs we use to treat them, which make these diseases far more difficult and expensive to treat.
Until recently, most doctors in the United States have not been checking pertussis samples for vaccine resistance. The authors of these findings recommend that more testing should be done across the United States “to determine whether our finding is a local event or represents a more widespread shift in B. pertussis strains.” They argue that this research will be key in the development of more effective vaccines.