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Herd Immunity

Some infections can be eradicated from the face of the planet. Smallpox is the one example of disease eradication to date. Smallpox still exists in US and Russian labs, but there has been no wild cases since 1977.  It is, like the Dorothy, history.

Why were we able to eradicate smallpox?  Three reasons:

1) There is only one form of smallpox. Unlike influenza that changes from year to year.  So only one vaccine needed.
2) By what appears to be a once in a universe miracle, every county cooperated with  the WHO (much like we all cooperate with the IRS) so the entire planet received the vaccine. Once enough people were vaccinated, the disease was unable to perpetuate itself and spread and so died out.
3) Unlike bacteria, there are no asymptomatic smallpox carrier states.  Eradicable viruses usually cause symptomatic disease and do not result in asymptomatic, infectious carrier states that serve as a reservoir for infecting others.  HIV and Herpes cause chronic asymptomatic infections and will probably never be eradicated.

There are other diseases that are theoretically eradicable, like measles and polio. They have one antigenic type, have no carrier state and, if the entire world could be vaccinated, the disease would cease to exist in the wild.  I am sure there would be biologic weapons labs that would always carry a vial or 2 of every infection. Just to be safe.

Could we ever eradicate bacterial diseases? No way. Not ever.  Bacteria will often colonize people, not causing disease.  Neisseria meningitis, for example, will, depending on the season and the population studied, will be found asymptomatically in the throat of up to 35% of people (1). The asymptomatic carriers serve as a source of bacteria that can subsequently be passed on to others, who, for reasons of genetics or bad luck, develop invasive disease.

Asymptomatic carriage can be important to developing immunity to bacteria.  Like vaccines, carriage exposes the immune system to small amounts of antigen and can lead to immunity. Unlike vaccines, there is the small, but real, chance that the bacteria will become invasive and kill the patient. Or jump to another and kill them.

The meningococcal vaccine is not one of the stellar vaccines.  It has modest efficacy, but may make the difference between life and death in some patients.   The meningococcal vaccine can decrease the chance of an individual having invasive disease or dying from the disease, but perhaps more importantly, the vaccine can markedly decrease the asymptomatic carriage rates in a population (2).

The decrease in the number of disease carriers is vital to the prevention of bacterial infections. Vaccines are never 100% effective. Some people are genetically unable to respond to the vaccine, some have immunodeficiencies that preclude receiving vaccines or developing a response to the vaccine, some haven’t gotten around to vaccination or are too young to receive a vaccine.  If you vaccinate a large number of people, besides preventing disease in an individual, it helps protect the vulnerable in a population. Vaccines prevent disease propagation.

A recent example of beneficial  effects of the vaccine mediated decrease in carriers occurred with the conjugate pneumococcal vaccine that is given to children.  The conjugated pneumococcal vaccine is directed against the 7 most common disease causing strains. Pneumococcus is a nasty bacteria, causing pneumonia, sepsis, and meningitis.

The use of the vaccine lead to a decrease in the incidence of meningitis of 64%  for the vaccine strains in children less than 2 years old, but, due to a general decrease in the carriage rates in the community, the rates of meningitis also dropped in the greater than age 65 group by 54% and a decrease in meningitis for all ages by 73% (4).  The use of the vaccine in children has also lead to the decrease  in invasive pneumococcal disease in adults (3).

Herd immunity at work.   Part of herd immunity functions to decrease the number of people in a population who carry the disease so that an at risk population are not exposed.  Part of herd immunity functions by preventing the spread of some, especially viral, diseases.  If there are not enough vulnerable people in a population, the disease cannot spread and perpetuate.  However this mechanism for herd immunity is less helpful with bacteria, which can colonize or cause less obvious disease.

Like pertussis.

Pertussis, whooping cough, is caused by a bacteria, Bordetella pertussis.   It infects the upper airway of children, causing obstruction and intractable coughing and vomiting after coughing.  Kids can cough themselves to death, unable to stop coughing to take in a breath.  The whoop of whooping cough occurs when the kids cough themselves blue and rapidly suck in air so they do not suffocate. If you vomit while trying to inhale, as occurs with pertussis, the child can suck vomit into their lungs, a bad thing as Eric ‘Stumpy Joe’ Childs proved.

There is a vaccine against Pertussis and it has been effective in helping to almost eradicate the disease in the US. World Wide disease there are about 294,000 deaths from pertussis.  In the US disease  “the rate of pertussis peaked in the 1930s, with 265,269 cases and 7518 deaths reported in the United States. This rate decreased to a low in 1976, when 1010 cases and 4 deaths occurred (9)”.  Before the vaccine pertussis killed about 8000 children a year, a death rate of about 1 in 500 (10).
Pertussis cases have been climbing. There were, in 2007, 10,000 cases of pertussis in the US, a new record.
USA. USA. USA.

Those numbers are all well and good, but a moving picture is worth thousands of words.
Warning: children are suffering in this video.
http://www.youtube.com/watch?v=dZ5jf-5MobE.

This is what the vaccines prevent.  This what will come back as vaccination rates fall.

The vaccine is good, but not perfect. Vaccine efficacy is  64%  for cases defined by mild cough,  81% for paroxysmal cough, and 95% for severe clinical illness (11). Note the vaccine is good for attenuating the disease, not preventing it entirely.  Patient with a cough are very infectious.  Cough is a great way to spread disease (15).  The reason the doctor asks you to turn your head and cough when testing for a hernia is not that turning the head improves the hernia exam, it is so you do not cough on the doctor, a remnant of the age of Tb.   And immunity wanes with time, so older populations are at increased risk for having asymptomatic  disease (8)

Pertussis persists in the adult population, due to declining immunity over  time and primarily presents as a prolonged cough, not whooping cough. Adults have enough immunity to avoid the severe manifestations of the disease.  And pertussis is common.

“From September 1986 through February 1989, we studied UCLA students with cough that lasted 6 days or more  . During this 2.5-year period, we found that 26% of the evaluated students had pertussis and that illness was endemic throughout the study period. Similar studies  done in adults in the United States, Australia, and Germany have had generally similar findings. Twelve percent to 32% of persons with prolonged cough have been found to have pertussis. In our study, important clinical findings in persons with pertussis were that the median duration of cough illness before seeking care was 21 days, productive cough was rare, the most common clinical diagnosis was bronchitis, and in no case was the diagnosis of pertussis entertained (9).”

So there is a huge potential source of pertussis, omnipresent, presenting atypically, at least as far as whooping cough is concerned, ready to kill.  Maximizing immunity in children and boosting immunity in adults is the only way to control pertussis:  Herd immunity.

With pertussis, while herd immunity may help prevent disease spread, because it is a bacteria and can be present without causing illness, the  herd immunity rates required to prevent the spread of disease are much higher than needed for viruses.  Immunity rates needed to protect the population from pertussis are about 94%, while virus spread is decreased if immunity rates are only 80% or so.  More of the population can be vaccine slackers and not be at risk for a viral illness, but not pertussis.

As discussed in this blog, there are pockets of non-vaccination in California.  According to the LA times, over 10,000 kindergartners had vaccine exemptions and some schools had very high no vaccine rates, many over 20%.

So far, no big outbreaks, but pertussis is the one I would expect to hit first.  As it requires the highest herd immunity rates and the has the biggest source of potential sources, California is primed for a resurgence of pertussis. All you would need to infect, say, an entire Waldorf school, is one adult who comes to work with a cough.

Published in Pediatrics this month is a paper that looked at the effect of vaccine refusers on laboratory confirmed (PCR or cultures) pertussis in Colorado (13).  They had 158 cases of pertussis  in the Kaiser system between 1996 and 2007.

Infected children were significantly more likely to have parents who refused vaccinations  (11.5%) than the controls (0.5%). The difference translated to a 22.8-fold increased risk of pertussis in the unvaccinated children.

At the time in Colorado, vaccination refusal was less than 1%, but they accounted for 11% of the cases of pertussis.  That’s not surprising.  With a bacterial disease like pertussis, a small slip in the vaccine rate can lead to a big jump in disease.

The vaccine is not 100%.  So there will be cases in vaccinated children as well.  What is striking is ALL the cases in the unvaccinated group could be attributed to not having the vaccine.

Oh, that’s Colorado.  I don’t need to worry. Doesn’t apply to my community.

There have been natural experiments in the world where diseases that were rare due to immunization came back with a decline in vaccination rates. Like Sweden. Watch as immunity in the population falls, watch as pertussis comes back. On the count of three, everyone say duh.

“Immunization against pertussis was introduced in Sweden in the 1950s and discontinued in 1979. This was followed by a low endemic level of pertussis for 3 years. Thereafter the incidence gradually increased and there were two outbreaks in 1983 and in 1985. In the period 1980 to 1985 pertussis was confirmed by culture or serology in 36,729 patients of which 11% were younger than 12 months of age and 69% were ages 1 to 6 years. An estimate of the total frequency of pertussis in preschool children was made from reports from a sample of the child health centers. The annual incidence rate per 100,000 population ages 0 to 6 years increased from the 700 cases in 1981 to 3200 in 1985. The ratio of total cases to those reported from the laboratories was 3:1 in 1981 and 2:1 in 1985. The cumulative incidence rate by the average age of 4 years was estimated at 16% of the unimmunized cohort born in 1980 compared with 5% of the immunized cohort born in 1978. The seriousness of pertussis was evaluated by studying the 2282 pertussis patients hospitalized from 1981 to the end of 1983. Forty-eight percent were infants younger than 12 months of age. Neurologic complications were noted in 4% and pneumonia in 14% of the hospitalized patients. Eleven children received assisted ventilation. Fatal outcomes were reported in 3 children (0.1%), 2 of whom had severe congenital disabilities (8).”

Ready?
One.
Two.
Three.

Hey.  I heard that all the way in Portland.

Similar outbreaks of diphtheria, a bacterial disease that requires viral herd immunity rates of about 85% to prevent spread.  When the Soviet Union fell apart, the vaccinations levels fell due to no health coverage, poor economics and a fear of vaccines perpetuated by anti vaccination proponent.

Part of the problem was

“changes in the immunization schedule during this period encouraged less intensive vaccination of children. Use of an alternative schedule of fewer doses of lower antigenic content (adult formulation) vaccine was allowed beginning in 1980.”

Good thing that doesn’t describe anyone in this country.  Who would be nuts enough to think that we give too may vaccines too soon?  Certainly no MD. That’s a path down which lies huge epidemics with horrific morbidity and mortality.

The old USSR  went from 3,000 diphtheria cases to 50,000 cases in 5 years  as vaccination rates fell from almost  universal coverage to 69% (7).  Diphtheria was not controlled until vaccination rates were pushed back into the low 90’s using the old schedule.

Vaccination rates are drifting down in some parts of the US. The bacteria and viruses are not gone and never will be.  As the vaccination rates fall, the herd loses its immunity and can no longer provide protection.  The risk is slowly building and there will be more outbreaks.  The reason to get vaccinated becomes increasingly compelling.  And only Jenny McCarthy will be satisfied if the epidemics return (6).
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References

1) FEMS Microbiol Rev. 2007 Jan;31(1):52-63.C  Lessons from meningococcal carriage studies.    PMID: 17233635

2) J Infect Dis. 2008 Mar 1;197(5):737-43    Impact of meningococcal serogroup C conjugate vaccines on carriage and herd immunity.  PMID: 18271745
3) Clin Infect Dis. 2009 Jan 1;48(1):57-64.    Epidemiology of invasive pneumococcal disease among adult patients in barcelona before and after pediatric 7-valent pneumococcal conjugate vaccine introduction, 1997-2007.   PMID: 19035779

4) Effect of pneumococcal conjugate vaccine on pneumococcal meningitis. N Engl J Med. 2009 Jan 15;360(3):244-56. PMID: 19144940

5) http://www.metrokc.gov/health/immunization/compare.htm

6) http://www.time.com/time/health/article/0,8599,1888718,00.html

7) http://www.cdc.gov/ncidod/eid/vol4no4/vitek.htm

8) Pediatr Infect Dis J. 1987 Apr;6(4):364-71.   Pertussis in Sweden after the cessation of general immunization in 1979.

9) http://www.annals.org/cgi/content/full/128/1/64

10) http://www.metrokc.gov/health/immunization/compare.htm

11) http://jama.ama-assn.org/cgi/content/abstract/267/20/2745

12) http://www.cdc.gov/mmwr/preview/mmwrhtml/00046738.htm

13)  Glanz J, et al “Parental refusal of pertussis vaccination is associated with an increased risk of pertussis infection in children” Pediatrics 2009; DOI: 10.1542/peds.2008-2150.

14)  http://www.cdc.gov/ncidod/eid/vol4no4/vitek.htm

15) Only sex is as efficient a way to spread infection.  If someone coughs on you during sex, it’s all she wrote.  Make sure your affairs are in order.

Posted in: Public Health, Science and Medicine, Vaccines

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