Tag: attack rate

How Contagious Is Measles?

Did you hear about the folks in New York who got quarantined isolated on the Emirates plane from Dubai?

Turns out that about 10 passengers had the flu or other cold viruses.
Although the worry was likely about MERS, it turns out that about 19 passengers had the flu or other cold viruses.

News like that and folks getting exposed to other infectious diseases, probably has them wondering just how contagious these diseases are. Do you have to be sitting next to someone to get them? In the same row? On the same floor?

Understanding Your Risk of Catching a Disease

Fortunately, most diseases are not terribly contagious.

We worry about some things, like SARS and Ebola, because they are so deadly, not because they are so contagious or infectious.

Wait, contagious or infectious? Aren’t they the same thing?

To confuse matters, some infectious diseases aren’t contagious, like Lyme disease. And some vaccine-preventable diseases are neither infectious nor communicable. Think tetanus. You may have never thought of it that way, but you aren’t going to catch tetanus from another person. Of course, that’s not a good reason to skip getting a tetanus shot!

To understand your risk of getting sick, you want to understand a few terms, including:

  • infectious disease – a disease that can be transferred to a new host
  • communicable – an infectious disease that can be transferred from one host to another
  • non-communicable – a non-infectious disease which can not be transferred from one host to another
  • contagiousness – an infectious disease that is easily transferred from one person to another
  • infectivity – the ability of an infectious agent to cause an infection, measured as the proportion of persons exposed to an infectious agent who become infected. Although this doesn’t sound much different from contagiousness, it is. The Francisella tularensis bacteria is highly infectious, for example, to the point that folks exposed to a culture plate are given antibiotics or put on a fever watch. Few of us get tularemia though, because transmission is through tick bites, hunting or skinning infected rabbits, muskrats, prairie dogs and other rodents, or inhaling dust or aerosols contaminated with F. tularensis bacteria. So if you get exposed, you will probably get sick, but there is a low probability for getting exposed.
  • incubation period – the time it takes to start having symptoms after you are exposed to an infectious disease. A longer incubation period increases the chances that someone will get exposed to a disease and travel home before getting sick. A shorter incubation period, like for influenza, means that a lot of people can get sick in a short amount of time.
  • contagious period- the time during which you can spread the illness to other people and may start before you have any symptoms
  • quarantine – used to separate people who have been exposed to a contagious disease and may become sick, but aren’t sick yet
  • isolation – used to separate people who are already sick with a contagious disease
  • transmission – how the disease spreads, including direct (direct contact or droplet spread) vs indirect transmission (airborne, vehicleborne, or vectorborne)
  • R0 (r nought) – the basic reproductive number or the number of new infections originating from a single infectious person among a total susceptible population
  • Rn – the net reproductive number, which takes into account the number of susceptibles in a community
  • infectious period – how long you are contagious

Got all that?

How Contagious Is Measles?

If not, understanding how easily you can get measles should help you understand all of these terms.

Measles is highly contagious, which is likely why all of the Brady kids got sick.
Measles is highly contagious, which is likely why all of the Brady kids got sick.

Measles is highly contagious, with a very high R0 number of 12 to 18.

That’s because:

  • the measles virus can live for up to two hours on surfaces and in the airspace where an infected person coughed or sneezed
  • infected people are contagious for up to four days before they have a rash and even know that they have measles, so expose lots of people even if they get put in isolation once they get diagnosed
  • infected people continue to be contagious for up to four days after the rash appears, so can continue to expose people if they aren’t put in isolation

So you don’t need to have someone with measles coughing in your face to get sick. If they coughed or sneezed at the grocery store, on the bus, or at your doctor’s office and then you entered the same area within two hours, then you could be exposed to the measles virus and could get sick.

Why don’t we see at least 12 to 18 people in each measles outbreak anymore?

That’s easy. The definition for R0 is for a total susceptible population. Most folks are vaccinated and protected, so even if they are around someone with measles, they typically won’t get sick.

Still, up to 90% of folks who aren’t immune and are exposed to measles will catch it. That includes infants too young to be vaccinated, kids too young to be fully vaccinated, and anyone who has a true medical exemption to getting vaccinated.

The measles has a very high R0 is easier to see when you compare it to those of some other diseases

 

Infection R0
Diphtheria 6-7
Ebola 1.5-2.5
Flu 1.4-4
MERS 2-8
Mumps 4.7
Pertussis 5-17
Polio 2-20
RSV 3
SARS 2-5
Smallpox 5-7
Varicella 8-10

Why such a big range for some diseases?

These are estimates and you are more or less contagious at different stages of each illness.

Fortunately, in most cases you can just get vaccinated and protected and don’t have to worry too much about them.

More on the Contagious Periods of Diseases

Is Mutating Mumps More Than the MMR Can Manage?

It is not news that we have been seeing more cases of mumps in recent years.

It is also isn’t news that many of these folks are vaccinated.

“Long Beach has been hit with a mumps outbreak that is vaccine-resistant. According to health officials in the Long Island town, almost two dozen individuals are believed to have contracted the virus, with four confirmed cases and at least 14 suspected ones.”

Natural News

That sites like Natural News is putting out misinformation about vaccine-resistant strains of mumps also shouldn’t be news to anyone.

Why Do Folks Think That Vaccine-Resistant Viruses Are Causing Mumps Outbreaks?

So are vaccine-resistant mumps viruses causing outbreaks?

There is no good evidence of that and plenty of evidence that our current vaccines, even though they aren’t perfect, do cover all wild strains of mumps.

Unfortunately, it might not be surprising that some folks are confused about vaccine-resistant mumps viruses, when we have health officials saying things like:

“Sometimes nature throws a strain at us that might have mutated a little bit, and coverage of the vaccine is not 100 percent.”

Dr. Lawrence Eisenstein, Nassau County Health Commissioner

Dr. Eisenstein’s “might have mutated a little bit” comment got twisted into “the outbreak is most likely attributable to a new strain of the virus that is resistant to vaccines” by health reporters

And out of Arkansas, where there have also been large mumps outbreaks:

“We are actually to the point that we are worried that this vaccine may indeed not be protecting against the strain of mumps that is circulating as well as it could.”

Dr. Dirk Haselow, Arkansas State Epidemiologist

Of course, to say that the vaccine may not be protecting folks “as well as it could” doesn’t mean it doesn’t work because the wild type mumps virus has evolved or mutated enough to surmount our current MMR vaccine.

Is Mutating Mumps More Than the MMR Can Manage?

Although anything is possible, we fortunately have plenty of research that says that the mumps virus hasn’t mutated and that the MMR still works.

During an outbreak, universities make sure students are up-to-date with their MMR vaccines.
During an outbreak of mumps, some kids are getting a third dose of the MMR vaccine.

In fact, although the MMR vaccine is made from the A strain or genotype of mumps, it provides good protection against all 12 known strains of wild mumps viruses, including genotype G that has been causing most of the recent outbreaks.

But how can it cover a different strain of virus that isn’t in the vaccine?

Because not all viruses and vaccines are like influenza.

“The genotyping of the mumps virus is based on the Small Hydrophobic (SH) protein, a nonstructural protein and genetically the most variable one. Based on the SH-protein 12 different mumps viruses were detected up to now. In recent epidemics in Western countries the genotype G was mainly detected, while the mumps viruses used in the live attenuated mumps vaccines belong to genotype A (Jeryl Lynn) and to a lesser extent to genotype B (Urabe). However, antibodies against the SH protein have not yet been observed in human serum. It is, therefore, unlikely that antibodies against the SH protein play an important role in antibody-mediated virus neutralization.”

Sabbe et al. on The resurgence of mumps and pertussis

It is well known that you need a very specific match of the flu vaccine to the wild flu virus that is going around to get good protection, but for many other viruses, the differences that determine the strain or genotype have nothing to do with how antibodies will recognize the virus.

“Since mumps virus is monotypic, vaccine from any strain should provide lifelong protection against subsequent infection.”

Palacios et al. on Molecular Identification of Mumps Virus Genotypes from Clinical Samples: Standardized Method of Analysis

Like measles, mumps is a monotypic virus.

“Studies have demonstrated that blood sera from vaccinated persons cross-neutralizes currently circulating mumps strains.”

CDC on Mumps for Healthcare Providers

And like measles, the mumps vaccine (MMR), protects against all strains of wild mumps viruses.

“Compared with attack rates of 31.8%–42.9% among unvaccinated individuals, attack rates among recipients of 1 dose and 2 doses of the Jeryl Lynn vaccine strain were 4%–13.6% and 2.2%–3.6%, respectively.”

Dayan et al. on Mumps Outbreaks in Vaccinated Populations: Are Available Mumps Vaccines Effective Enough to Prevent Outbreaks?

And like other vaccines, the mumps vaccine (MMR) works.

Waning immunity may be an issue, but that certainly isn’t a reason to skip or delay this vaccine and put your kids, and everyone else, at risk to get mumps.

What to Know About Mumps Strains and Outbreaks

The MMR vaccines covers all strains of mumps and getting fully vaccinated is the best way to make sure your kids don’t get mumps.

More on Mumps Strains and Outbreaks

Do More Vaccinated or Unvaccinated Kids Get Sick in Outbreaks?

Some anti-vaccine folks continue to claim that vaccines don’t work and that most outbreaks of vaccine-preventable diseases are actually caused by kids who have been vaccinated.

They also push the myth that more vaccinated than unvaccinated kids get sick in most outbreaks.

Vaccinated vs Unvaccinated in an Outbreak

So are outbreaks usually caused by kids who have been vaccinated?

No, of course not.

Do we sometimes see more vaccinated than unvaccinated kids in some of these outbreaks?

Yes, sometimes we do.

Vaccine Epidemiology

Wait, what?

Yes, we sometimes see more vaccinated than unvaccinated kids in an outbreak.

How can that be if vaccines work?

It is actually very easy to understand once you learn a little math and a little more epidemiology.

Basically, it is because while vaccines work, they don’t work 100% of the time, and more importantly, there are way more vaccinated kids around than unvaccinated kids.

The Mathematics of Disease Outbreaks

That means that you need to understand that more than the absolute number of vaccinated and unvaccinated people that got sick in an outbreak, you really want to know the percentages of vaccinated vs unvaccinated kids who got sick.

For example, in a school with 1,000 kids, you might be very surprised if six kids got a vaccine preventable disease, and three of them were vaccinated, leaving three unvaccinated.

Does that really mean that equal amounts of vaccinated and unvaccinated kids got sick?

I guess technically, but in the practical sense, it only would if half of the kids in the school were unvaccinated. Now unless they go to a Waldorf school, it is much more likely that over 90 to 95% of the kids were vaccinated, in which case, a much higher percentage of unvaccinated kids got sick.

Before we use a real world example, some terms to understand include:

  • attack rate – how many people will get sick when exposed to a disease
  • basic reproductive number or Ro – different for each disease, Ro basically tells you  just how contagious a disease is and ranges from about 1.5 for flu, 8 for chicken pox, and 15 for measles
  • vaccine coverage – how many people are vaccinated
  • vaccine efficacy – how well a vaccine works

You also need to know some formulas:

  • attack rate = new cases/total in group
  • vaccine coverage rate = number of people who are fully vaccinated / number of people who are eligible to be vaccinated
  • vaccine effectiveness = (attack rate in unvaccinated group – attack rate in vaccinated group) / attack rate in unvaccinated group x 100

Unfortunately, it is often hard to use these formulas in most outbreaks.

Why?

For one thing, it is hard to get accurate information on the vaccination status of all of the people in the outbreak. In addition to those who are confirmed to be vaccinated or unvaccinated, there is often a large number who’s vaccination status is unknown. And even if you know the vaccination status of everyone in the outbreak, it can be even harder to get the vaccine coverage rate or a neighborhood or city.

Outbreaks of Vaccine Preventable Diseases

Reports of measles outbreaks among highly vaccinated populations are from before we started doing a second dose in 1994.
Reports of measles outbreaks among highly vaccinated populations are from before we started doing a second dose of MMR… way back in 1994.

We know what starts most outbreaks.

And no, it’s not shedding

For example, with measles, it is typically an unvaccinated person who travels out of the country, returns home after they have been exposed but are still in their incubation period, and then exposes others once they get sick. And the great majority of folks in these measles outbreaks are unvaccinated.

Some examples of these outbreaks include:

  • the 2014 Ohio measles outbreak that started with two unvaccinated Amish men getting measles in the Philippines while on a missionary trip and ended up with at least 388 cases before it was over, almost all unvaccinated
  • a 2013 North Carolina measles outbreak with 22 cases started after an unvaccinated traveler had returned from India
  • an outbreak of measles in New York, in 2013, with at least 58 cases, tarted with an intentionally unvaccinated teen returning from a trip to London
  • a 2011 outbreak of measles in Minnesota, when an unvaccinated child traveled out of the country, developed measles, and returned to his undervaccinated community, causing the state’s largest measles outbreak in 20 years

But what about mumps and pertussis?

Those outbreaks are all among vaccinated kids, right?

Nope.

In one of the biggest mumps outbreak, in Arkansas, only 71% of people were up-to-date on their vaccines!

And keep in mind that while we do know that there are issues with waning immunity with some vaccines, you are still much more likely to become infected and get others sick if you are not vaccinated. And you will likely have a much more severe disease.

A 2013 pertussis outbreak in Florida is a good example that even with all the bad press it gets, the DTaP and Tdap vaccines work too. This outbreak was started by an unvaccinated child at a charter school with high rates of unvaccinated kids. About 30% of unvaccinated kids got sick, while there was only one case “in a person who reported having received any vaccination against pertussis.”

In another 2013 pertussis outbreak in Florida, this time in a preschool, although most of the kids were vaccinated, the outbreak started with “a 1-year-old vaccine-exempt preschool student.” And the classroom with the highest attack rate, was “one in which a teacher with a laboratory-confirmed case of pertussis who had not received a Tdap booster vaccination, worked throughout her illness.”

In outbreak after outbreak, we see the same thing, sometimes with deadly consequences – an unvaccinated child or adult triggers an outbreak and then a lot of unvaccinated folks get sick. Unfortunately, others get caught up in these outbreaks too, including those too young to be vaccinated, those who can’t be vaccinated because of true medical exemptions, and those whose vaccines may not have worked as well as we would have liked.

Get educated.

Vaccines are safe. Vaccines are necessary. Vaccines work.

What to Know About Vaccinated vs Unvaccinated in Outbreaks

Most outbreaks are started by someone who is unvaccinated, often after a trip out of the country, and the resulting outbreak will likely get many more unvaccinated than vaccinated folks sick.

More About Vaccinated vs Unvaccinated in Outbreaks