Tag: pertussis

Can Vaccinated Children Be Asymptomatic Carriers of Pertussis?

Why do anti-vaccine folks like to talk about baboons so much?

“Did you know that a study showed that baboons injected with whooping cough vaccine became infected with whooping cough anyway – and silently spread the disease to other baboons for 35 days?”

Anti-vaccine Meme

Is it because baboons are used in the study of vaccines?

That’s part of it, at least when they can find a study where they can cherry pick the results to suit their needs.

The Baboon Study

Like most anti-vaccine talking points, this one about baboons, has some truth to it.

An adult male baboon.
An adult male baboon. Photo by Elizabeth Miller

There was a baboon study with the pertussis vaccine and it found that previously vaccinated baboons could develop asymptomatic carriage of the pertussis bacteria after they were intentionally infected.

Here is where it is important to note that an infection is different than a disease.

The example that many people are familiar with is tuberculosis. It is common to have a TB infection without any signs or symptoms and to not feel sick. The only reason we know that they have TB is because they had a positive TB test.

Unfortunately, about 5 to 10% of these people with TB infections can eventually develop TB disease, with coughing, weight loss, night sweats, fever, and chest pain, etc.

It is kind of the same with the baboons in the study. Twenty-four hours after two previously vaccinated baboons were inoculated with pertussis bacteria in the back of their nose and trachea, an unvaccinated baboon was put in each of their cages.

The vaccinated baboons continued to have pertussis bacteria in their noses, which the researchers had put there, for up to 35 days. And they were able to eventually pass the pertussis bacteria to the unvaccinated baboons in their cages. Vaccinated baboons also became infected or colonized after they were put in a cage with an intentionally infected unvaccinated baboon.

“…animals did not cough and showed no reduction of activity, loss of appetite, or other outward signs of disease.”

Warfel et al on Acellular pertussis vaccines protect against disease but fail to prevent infection and transmission in a nonhuman primate model

The vaccinated baboons were infected, but they never did develop symptoms of pertussis.

What Does The Baboon Study Mean?

One thing that is for sure – the baboon study found that the pertussis vaccines work. Only unvaccinated baboons got sick with pertussis.

But does this study explain our current outbreaks of pertussis?

Are vaccinated people becoming colonized and then getting others sick?

I guess it is possible, but we are not baboons in a cage with other baboons. How would we spread a respiratory disease, even if we did become colonized with the bacteria, if we don’t have symptoms?

It may explain part of our outbreaks though.

If vaccinated people do commonly become colonized with pertussis bacteria, then they might very well test positive for pertussis even though they don’t have symptomatic pertussis disease. So when they develop a cold or bronchitis and are found to have a positive pertussis test, then couldn’t that test just indicate that they have a pertussis infection and not disease, even though something else is actually causing their symptoms?

That’s what we think happens with strep carriers, right?

That’s kind of what the baboon study found. All of the baboons tested positive, but only the unvaccinated baboons had symptomatic pertussis disease.

“Baboons vaccinated with wP vaccines exhibit a level of protection that is intermediate between convalescent animals and aP-vaccinated animals. They exhibit no outwards signs of disease and are initially colonized to the same high level as aP-vaccinated animals but clear the infection more rapidly.”

Pinto et al on Pertussis disease and transmission and host responses: insights from the baboon model of pertussis.

It is interesting to note that the baboon study also found that baboons who had received whole cell pertussis vaccines also became carriers. They just didn’t stay carriers for as long as the baboons who got the newer acellular pertussis vaccine. But since they were still carriers, if asymptomatic transmission is such a big problem, wouldn’t it have been a big problem back in the day when everyone got whole cell pertussis vaccines?

The Debate Over Asymptomatic Carriage

Most vaccines prevent the spread of disease.

Do the pertussis vaccines?

Most folks still think so.

“The baboon model pioneered by Warfel et al. is without question a game-changer, shedding light on the impact of vaccination on disease and infection. However, the view it affords is clearer with respect to immunity and pathology than with respect to transmission. We point out that the extrapolation of the possibility of transmission from vaccinated baboons in the laboratory to the probability of transmission from vaccinated humans in the population is unwarranted. More work is needed to elucidate the relative transmissibility of infections in vaccinated vs. unvaccinated hosts. The evidence adduced above suggests, however, that vaccination with aP must have a strong effect on transmission as well as disease.”

Matthieu Domenech de Cellès et al on Epidemiological evidence for herd immunity induced by acellular pertussis vaccines

Even the author of the baboon study has said that “We agree that these data should not be directly extrapolated to pertussis transmission in humans. Although baboons are >96% genetically similar to humans, there are likely differences in how the species respond to vaccination and infection. We also agree that aP-vaccinated infected people are likely less efficient at transmitting pertussis compared with unvaccinated infected people, although it is not clear to what extent.”

Others think that asymptomatic carriage of pertussis might behind a lot of our recent outbreaks. Or at least what helps them grow so large.

Still, it is important to remember that unvaccinated folks do play a role in these outbreaks too. In a pertussis outbreak at a Florida preschool, in which most kids were vaccinated, the outbreak was started by a vaccine-exempt toddler.

And we have seen this in many other areas and it has been confirmed by many studies. Whatever else is contributing to pertussis outbreaks, like waning immunity, they are also associated with vaccine refusal.

“Counties with higher exemption rates had higher rates of reported pertussis among exempted and vaccinated children when compared with the low-exemption counties.”

Imdad et al. on Religious exemptions for immunization and risk of pertussis in New York State, 2000-2011.

But what if the DTaP and Tdap vaccines do cause folks to be asymptomatic carriers?

Even if that is true, understand that these vaccines don’t actually infect you, making you a carrier. They just might not prevent you from becoming a carrier if you are exposed to someone else with pertussis. While that might be a good reason to develop a new and better pertussis vaccine, it certainly isn’t a reason to skip or delay your child’s vaccines now.

Remember that even with our current outbreaks, rates of pertussis were much higher in the pre-vaccine era.

What to Know About Vaccines and Asymptomatic Carriers of Pertussis

The role of asymptomatic carriers and pertussis is controversial, but it certainly isn’t a reason to skip or delay your child’s vaccines.

More on the Vaccines and Asymptomatic Carriers of Pertussis

Pertussis Outbreaks

Like measles and mumps, pertussis, or whooping cough, is another vaccine-preventable disease that has been increasing in recent years.

Pre-Vaccine Era Pertussis Outbreaks

Pertussis has been known since at least the Middle Ages, although the bacteria that causes pertussis, Bordetella pertussis, wasn’t discovered until 1906.

Like measles, pertussis was a top killer of young children in the pre-vaccine era.
Like measles, pertussis was a top killer of young children in the pre-vaccine era.

That discovery led to the later development of the first pertussis vaccines, but before then, pertussis was a big killer, with epidemic cycles every 2 to 5 years.

During one of these cycles in the United States, from 1926 to 1930, there were:

  • 909,705 cases, and
  • 36,013 deaths

Unfortunately, even natural infection doesn’t provide life-long immunity, so adults would get pertussis and give it to susceptible kids, who were most likely to die during these epidemics.

But even in non-epidemic years, a lot of folks got pertussis. The number of reported cases ranged from “just” 161,799 in 1928 to 202,210 in 1926. And during one of the biggest years, 1934, there were 265,269 cases!

Post-Vaccine Era Pertussis Outbreaks

That changed in the vaccine era.

The first pertussis vaccines were developed in the 1930s and became more widely used in the 1940s when it was combined into the whole-cell DTP vaccine.

This was replaced with the acellular DTaP vaccine in 1997, with the Tdap vaccine being added to the vaccine schedule in 2006.

These vaccines helped to greatly reduce how many people got pertussis and how many people died from pertussis:

  • 1940 – 183,866 cases
  • 1950 – 120,718 cases and 1, 118 deaths
  • 1960 – 14,809 cases and 118 deaths
  • 1970 – 4,249 cases and 12 deaths
  • 1980 – 1,730 cases and 11 deaths
  • 1990 – 4,570 cases and 12 deaths
  • 2000 – 7,867 cases and 12 deaths
  • 2010 – 27,550 cases and 26 deaths

They never eradicated pertussis though, and as you can see, recently, pertussis cases have started to rise again.

Is it a coincidence that whooping cough came back as more folks began to skip and delay vaccines for their kids?
Is it a coincidence that whooping cough came back as more folks began to skip and delay vaccines for their kids?

In 2012, there were 48,277 cases of pertussis in the United States, the most since 1950, when we had 68,687 cases. Unfortunately, with the rise in cases, we are also seeing the tragic consequences of this disease – 20 deaths in 2012, mostly infants under age 3 months.

Pertussis cases remained steady, but high, in 2013 and 2014, at around 30,000 cases in the United States.

In California, pertussis reached epidemic levels. The California Department of Public Health reported at least 11,114 cases in 2014 – the highest numbers of pertussis cases in the state in 70 years!

And as expected with the rise in cases, there were 3 pertussis related deaths in California that year – all infants who had contracted pertussis when they were less than 8 weeks old. Two of the infants became sick in 2013, but the third, a 5-week-old baby, got infected in 2014.

Another baby, only 25 days old died in early 2015, but will be counted as the 2nd death of 2014 since that is when the illness started. About 383 patients, mostly infants who are less than 4 months old, were hospitalized in California that year, including 80 who required intensive care. And according to the California Department of Public Health, about 82% of the cases in infants were born to mothers who did not receive a dose of Tdap during their third trimester of pregnancy.

What’s happened since then?

Pertussis cases are continuing to fall each year! In fact, with about 16,000 cases in the United States, 2017 may have ended with the lowest number of pertussis cases since 2008.

Still, with just 1,830 pertussis cases in California in 2016, there were two deaths – both infants who were younger than 3 months of age when they got sick. And there was at least one death in 2017, with similar rates of disease, although reports are still preliminary.

Why So Many Pertussis Outbreaks?

Ever since a 2010 California pertussis outbreak, in which there were 9,154 cases of pertussis, the most in 63 years, and 10 infants died, many people, especially parents, began wondering why we were seeing more pertussis these days.

Is it because the pertussis vaccines simply don’t work, as the anti-vaccine movement would have you think?

Or is it because there are higher rates of unvaccinated kids these days and parents using alternative immunization schedules, instead of the standard immunization schedule from the CDC?

James Cherry, MD is an expert on pertussis and pertussis vaccines.
James Cherry, MD is an expert on pertussis and pertussis vaccines.

A commentary, Why Do Pertussis Vaccines Fail?, by James Cherry, MD, gave us some answers.

While the title of the article might have you think that all of the blame lies with the pertussis vaccines, that certainly isn’t the case. While there can be vaccine failures with the pertussis vaccines, just like any other vaccine, that doesn’t mean that the vaccine doesn’t work for most children.

One of the problems is that the DTaP vaccine likely isn’t as effective as the older DTP vaccine. So instead of efficacy of 84 to 85%, as was once believed, it is likely closer to just 71 to 78%.

Other issues, including waning immunity, the possibility of an incorrect balance of antigens in the vaccine that could create a blocking effect, and genetic changes in the B. pertussis bacteria, could also possibly lead to increased vaccine failure rates.

So it isn’t that the pertussis vaccines don’t work.

That should be easy to see when you look at the pertussis rates in California, when the highest rates by far were in infants less than 6 months of age (434 per 100,000 people). In contrast, children who were 6 months to 6 years old had a rate of only 62 per 100,000.

And the results of a study that were presented at the 49th annual meeting of the Infectious Diseases Society of America in Boston show just how important the pertussis vaccine is, as:

  • vaccine effectiveness was 98.1 percent among children who received their 5th dose within the past year
  • long term effectiveness – children who were five or more years past their last DTaP dose – was about 71 percent
  • children who had never received any doses of DTaP (unvaccinated children) faced odds of having whooping cough at least eight times higher than children who received all five doses

It is also important to note that the high rates seen in 2010 in California are still well below the rates that were seen in the pre-vaccination era, when the attack rate of pertussis in the United States was as high as 157 per 100,000 people, with about 200,000 cases a year.

What’s the answer?

“The present “resurgence of pertussis” is mainly due to greater awareness and the use of PCR for diagnosis. There are also many other factors which have contributed to the “resurgence.” New vaccines are clearly needed; with our present vaccines (DTaP and adolescent and adult formulated tetanus and diphtheria toxoids and acellular pertussis vaccine (Tdap)), if used correctly, severe pertussis and deaths in infants can be prevented.”

James D. Cherry, MD on The History of Pertussis (Whooping Cough); 1906 – 2015: Facts, Myths, and Misconceptions

It certainly isn’t for more kids to follow non-standard, parent-selected, delayed protection vaccine schedules or to simply skip vaccines all together. Since natural immunity isn’t going to keep newborns and infants from getting pertussis, the ages which are most at risk for life-threatening infections, they can catch pertussis from people around them, including those working on their natural immunity. Natural infections don’t even provide life-long protection against pertussis, as some people believe. That natural immunity wanes fairly quickly too.

Not Vaccinated? No Kisses!
Not Vaccinated? No Kisses!

The future of pertussis control is more likely going to be in maximizing our current vaccination program, including getting more teens and adults to get the Tdap vaccine, especially when women are pregnant.

That’s the best strategy, at least until new pertussis vaccines are developed. It provides a lot of benefits. According to the CDC, like with the flu vaccine, when you get a pertussis vaccine, in addition to protecting yourself and those people around you, “people who do catch whooping cough after being vaccinated are much less likely to be hospitalized or die from the disease.”

Unfortunately, not everyone has gotten the message. And because of waning immunity, children who aren’t vaccinated against pertussis can’t “hide in the herd” and rely on the rest of us who do vaccinate our children to provide them with protection. Instead, since they are at a higher risk, they get pertussis and get even more people sick.

This slogan, during a whooping cough epidemic, reminded parents to get their kids vaccinated now.
This slogan, during a whooping cough epidemic, reminded parents to get their kids vaccinated now.

In one study, Parental refusal of pertussis vaccination is associated with an increased risk of pertussis infection in children, researchers found that “vaccine refusers had a 23-fold increased risk for pertussis when compared with vaccine acceptors, and 11% of pertussis cases in the entire study population were attributed to vaccine refusal.” The highly contagious nature of pertussis then means every primary case is probably going to infect as many as 17 other people. That’s why it makes sense that higher rates of children using vaccine exemptions could be at least one of the factors in these outbreaks.

In fact, several studies, including, Geographic Clustering of Nonmedical Exemptions to School Immunization Requirements and Associations With Geographic Clustering of Pertussis, found that “geographic pockets of vaccine refusal are associated with the risk of pertussis outbreaks in the whole community.”

Get educated. Vaccines are safe and as you can see with the rise in outbreaks, vaccines are necessary.

What to Know About Pertussis Outbreaks

Many factors are responsible for the rise in pertussis outbreaks in recent years, but it is clear that being unvaccinated and unprotected put you at greatest risk for getting pertussis and passing it on to others.

More on Pertussis Outbreaks

What Are Hypotonic-Hyporesponsive Episodes?

Just about any side effect after a vaccine can be scary for parents.

What if your child suddenly became limp, wasn’t responsive, and was pale?

That would be scary for any parent.

What Are Hypotonic-Hyporesponsive Episodes?

But that’s just what can happen when a child has a hypotonic–hyporesponsive episode (HHE).

“A hypotonic-hyporesponsive episode (HHE) is the sudden onset of hypotonia, hyporesponsiveness, and pallor or cyanosis that occurs within 48 hours after childhood immunizations.”

DuVernoy et al on Hypotonic-hyporesponsive episodes reported to the Vaccine Adverse Event Reporting System (VAERS), 1996-1998

These types of episodes were once thought to happen once for every 1,750 DTP vaccines given.

HHE is much more rare since we switched to a new pertussis vaccine.
HHE is much rarer since we switched to a new pertussis vaccine.

Fortunately, although they certainly do sound scary, the episodes stop on their own and  don’t cause any permanent harm.

Hypotonic-hyporesponsive episodes were even removed as table injuries after DTP back in 1995. It is not that HHE can’t occur after DTP, DTaP, or other vaccines, but rather that HHE doesn’t then cause any permanent neurological damage to the child.

And it is rare for kids to have a second episode, so they can continue to get vaccinated. HHE is not a good reason to skip or delay all of your child’s vaccines. While not a contraindication to getting vaccinated, having an episode of HHE “within 48 hours after receiving a previous dose of DTP/DTaP,” is listed as a precaution to getting another dose of DTaP or Tdap though.

“In general, vaccinations should be deferred when a precaution is present. However, a vaccination might be indicated in the presence of a precaution if the benefit of protection from the vaccine outweighs the risk for an adverse reaction.”

CDC on Vaccine Contraindications and Precautions

Also, HHE has become even more rare since we switched to using DTaP, instead of the older DTP vaccine. So being worried about HHE is definitely not a good reason to skip or delay any vaccines.

What to Know About Hypotonic-Hyporesponsive Episodes

Hypotonic-hyporesponsive episodes were more common after the older DTP vaccines, but still didn’t cause any long term problems and aren’t a good reason to skip or delay your child’s vaccines.

More About Hypotonic-Hyporesponsive Episodes

Challenging the Concept of Herd Immunity

Before we talked about clusters of unvaccinated children, experts warned about pockets of susceptibles.
Before we talked about clusters of unvaccinated children, experts warned about pockets of susceptibles.

The idea of herd immunity has been around since at least 1923 and became used to describe “the indirect protection afforded to individuals by the presence and  proximity of others who are immune.”

That’s not much different from how the CDC defines herd immunity today:

A situation in which a sufficient proportion of a population is immune to an infectious disease (through vaccination and/or prior illness) to make its spread from person to person unlikely. Even individuals not vaccinated (such as newborns and those with chronic illnesses) are offered some protection because the disease has little opportunity to spread within the community.

Also called community immunity, it is often misunderstood by folks in the anti-vaccine movement.

Challenging the Concept of Herd Immunity

That the idea of herd immunity is being challenged is not new.

“Along with the growth of interest in herd immunity,  there has been a  proliferation of views of what it means or even of whether it exists at all.”

Paul E. M. Fine Herd Immunity: History, Theory, Practice

If you get educated about vaccines and understand how herd immunity works, it is easy to refute these challenges, especially the idea that herd immunity isn’t real just because we still have outbreaks of vaccine-preventable diseases among highly vaccinated communities.

Why do we still have outbreaks then?

It is mostly because we live in open communities that don’t mix randomly.

Keep in mind that the best model for herd immunity is a randomly mixing closed community – “one in which the probability of contact within any time interval is the same for every choice of two individuals in the population.”

Again, that doesn’t mean herd immunity doesn’t work.

It just means we can expect to see some outbreaks when someone in a well vaccinated community visits another community with lower vaccination levels and more disease, gets sick, and returns.

“However,  within the population of a community,  there may be pockets of  susceptibles, either because prior epidemics have failed to spread into the group or because they have not accepted immunization.”

John P. Fox Herd Immunity

You must also consider the size of the community when thinking about herd immunity, for example, a family, school, neighborhood, or city, versus the entire state. So you can have herd immunity levels of protection at the state or city level because of high average vaccination levels, but pockets of susceptibles who live in the same neighborhood or go to the same school can mean that you don’t have herd immunity in those places, leading to outbreaks.

“Hib vaccine coverage of less than 70% in the Gambia was sufficient to eliminate Hib disease, with similar findings seen in Navajo populations.”

RA Adegbola Elimination of Hib disease from The Gambia after…

Lastly, there is not one herd immunity level for all diseases. It is a separate threshold for each and every disease, depending on how easily it spreads, how many people are already immune, how long immunity lasts, if there is a vaccine, and the effectiveness of the vaccine, etc. That means that a community can have herd immunity for Hib and polio, but not the flu, and for rubella and measles, but not pertussis.

What happened in The Gambia is a great example of herd immunity. After introducing a three dose primary Hib immunization schedule (no booster dose), rates of Hib meningitis quickly went from 200 per 100,000 to none. A few years later, there were 6 cases of Hib meningitis in mostly vaccinated children (no booster dose) and in the majority of cases, “close contacts had a history of frequent or recent travel to Senegal, a neighboring country with strong kinship links with The Gambia and where vaccination against Hib was not introduced” until the following year.

With a Hib meningitis rate of 3 per 100,000, they are still far below pre-vaccine levels of disease, and their situation doesn’t mean that herd immunity isn’t real, as you will understand once you review these myths about herd immunity.

Myths About Herd Immunity

What are some common myths about herd immunity?

  • that natural immunity is better than getting vaccinated. Not True. Natural immunity often comes with a price. Remember, many vaccine-preventable diseases are life-threatening, even in this age of modern medicine.
  • you can just hide in the herd. Not True. “Freeloaders” can gamble and hope that their intentionally unvaccinated kids won’t get a vaccine-preventable disease, but it won’t always work. There is a risk to “free-riding, in which individuals profit from the protection provided by a well-vaccinated society without contributing to herd immunity themselves.”
  • most adults aren’t immune because they haven’t been vaccinated or don’t get boosters, but since we aren’t seeing that many outbreaks, herd immunity itself must be a myth. Not True. Adults were either born in the pre-vaccine era and likely have natural immunity or were born in the vaccine era and are vaccinated and immune. But again, herd immunity is disease specific, so when we talk about herd immunity for measles, it doesn’t matter if they have immunity against hepatitis A or Hib. And adults get few boosters or catch-up vaccines. Also, some vaccines, like Hib and Prevnar, have indirect effects, protecting adults even though they aren’t vaccinated because vaccinated kids are less likely to become infectious.
  • most vaccines wear off too soon to provide long lasting protection for herd immunity to be real. Not True. While waning immunity is a problem for a few diseases, like pertussis and mumps, and you need boosters for others, like tetanus, vaccine induced immunity is typically long lasting and often life-long.
  • herd immunity wasn’t developed by observing immunized people, it was all about natural immunity. Not True. The first experiments about herd immunity by Topley and Wilson in 1923 involved vaccinated mice. Ok, they weren’t immunized people, but it wasn’t just about natural immunity! And much earlier, in 1840, it was noted that “smallpox would be disturbed, and sometimes arrested, by vaccination, which protected a part of the population.” That’s herd immunity he was talking about.
  • herd immunity is not a scientifically validated concept. Not True. It has been well studied for almost 100 years.
  • if herd immunity was real, diseases would be eradicated once you reached herd immunity levels. Not True. Reaching herd immunity levels simply starts a downward trend in disease incidence. A little more work has to be done at the final stages of eradication, like was done for smallpox and is being done for polio.
  • natural immunity causes much of the decrease in mortality from a disease in the developed world, even before a vaccine is introduced. Not True. While it is certainly true that there was a big drop in mortality in the first half of the 20th century for most conditions because of improvements in sanitation, nutrition, and medical science, it was not a consequence of natural herd immunity. And we continue to see significant levels of mortality and morbidity for many diseases in the modern era, especially for those that can’t yet be prevented by a vaccine, like RSV, West Nile Virus, and malaria, etc.
  • vaccines aren’t 100% effective, so herd immunity can’t really work. Not True. Part of the equation to figure out the herd immunity threshold for a disease takes into account the effectiveness of a particular vaccine.
  • folks with medical exemptions for vaccines put the herd at risk just the same as those who intentionally skip vaccines. Not True. Children and adults with medical exemptions, including immune system problems, those getting treatments for cancer, and other true medical exemptions don’t have a choice about getting vaccinated.

So, like other anti-vaccine myths, none of the herd immunity myths you may have heard are true.

That makes it hard to understand why Dr. Russel Blaylock goes so far as to say “that vaccine-induced herd immunity is mostly myth can be proven quite simply.” Does he just not understand herd immunity? That is certainly a possibility, because “although herd immunity is crucial for the elimination of infectious diseases, its complexity and explicit relationship to health politics cause it to remain under-explained and under-used in vaccine advocacy. ”

He is also really big into pushing the idea that adults have no or little immunity, because when he was in medical school, he was “taught that all of the childhood vaccines lasted a lifetime,” but it has now been discovered that “most of these vaccines lost their effectiveness 2 to 10 years after being given.”

The thing is, Blaylock graduated medical school in 1971, when the only vaccines that were routinely used were smallpox (routine use ended in 1972), DPT, OPV, and MMR (it had just become available as a combined vaccine in 1971). Of these, it was long known that smallpox, diphtheria, and tetanus didn’t “last a lifetime,” and the live vaccines OPV and MMR, except for the mumps component, actually do.

Blaylock, like most anti-vaccine folks who push myths about herd immunity, is plain wrong. And like most anti-vaccine myths, using herd immunity denialism to convince parents that it is okay to skip or delay vaccines puts us all at risk for disease.

What To Know About Herd Immunity Myths

Herd immunity is not junk science or a false theory. Herd immunity is real, it works, and explains how people in a community are protected from a disease when vaccination rates are above a certain threshold.

More About Herd Immunity Myths