Herd Immunity and the Varicella vaccine

[carriebft]

So I thought it might be an interesting idea to go through a few vaccines and see what the research says about herd immunity and that vaccine.

let's start with varicella.

How effective is it at time of vaccination?
Does it prevent transmission?
Does immunity wane?
is the 'herd' protected at a certain vaccination level?

[carriebft]

I'll start with a few studies I have collected:

http://content.nejm.org/cgi/content/...ct/347/24/1909

Conclusions In this outbreak, vaccination provided poor protection against varicella, although there was good protection against moderate or severe disease. A longer interval since vaccination was associated with an increased risk of vaccine failure. Breakthrough infections in vaccinated, healthy persons can be as infectious as varicella in unvaccinated persons.

7 year immunity study:

http://cat.inist.fr/?aModele=afficheN&cpsidt=14150392


another immunity duration study:
http://jama.ama-assn.org/cgi/content/abstract/291/7/851

Conclusions Although varicella vaccine is effective, its effectiveness decreases significantly after 1 year, although most cases of breakthrough disease are mild. If administered at younger than 15 months, the vaccine's effectiveness was lower in the first year after vaccination, but the difference in effectiveness was not statistically significant for subsequent years.

[shuttlt]

Do you happen to know the herd immunity threshold for this one?

[mamakay]

Originally Posted by shuttlt

Do you happen to know the herd immunity threshold for this one?

110%.

(vaccine critical site, but an excellent, well referenced article)

http://insidevaccines.com/wordpress/...-sure-why-not/

With varicella, once circulation of the organism dies out from a lack of non-immune hosts, it starts re-emerging in the form of singles in it's former hosts.

[shuttlt]

As to the benifits of an immune herd:

It has been estimated that varicella complications occur in 5% to 10% of healthy children(4). Complications include skin and soft tissue infections, group A streptococcal infections, otitis media, pneumonia, and encephalitis. Between 0.2% and 1.5% of Canadian children with varicella are admitted to hospital(4). Maternal varicella during the first and second trimesters of pregnancy can lead to congenital varicella syndrome (0.4% to 2% risk)(4) ; maternal varicella around the time of delivery can result in severe neonatal varicella in 17% to 30% of infants(4). From 1987 to 1996, there were one to 16 deaths per year attributable to varicella in Canada; 70% were of individuals > 15 years old(4).

http://www.phac-aspc.gc.ca/publicat/.../dr2819ea.html

One thing that interests me is that the same article says: "By 12 years of age, over 90% of Canadians have been infected and have developed lifelong immunity to the virus". Let's say the rate at which the population is exposed to the disease is uniform with respect to age. It should be possible to work out how many pregnant women would be expected to get the virus and how many cases of congenital varicella syndrome there should be.

[shuttlt]

Originally Posted by mamakay

110%.

(vaccine critical site, but an excellent, well referenced article)

http://insidevaccines.com/wordpress/...-sure-why-not/

With varicella, once circulation of the organism dies out from a lack of non-immune hosts, it starts re-emerging in the form of singles in it's former hosts.

But if it needs 110% you can't have herd immunity and the organism is only ever going to die out locally.

[anewmama]

Originally Posted by shuttlt

Do you happen to know the herd immunity threshold for this one?

As my daughter says, "Was Dat?"

[mamakay]

Originally Posted by shuttlt

But if it needs 110% you can't have herd immunity and the organism is only ever going to die out locally.

It's not even going to die out locally, really. Imagine if once measles stopped circulating in the US, we all satarted spontaneously generating the measles virus in our own bodies, spreading the virus around that way. What kind of herd immunity is that, really?

That's what we're facing with varicella because of it's adaptation to normal "herd immunity". You never completely clear the virus, it just goes into latency in the dorsal root ganglia, and if a certain amount of time passes in the host without the host being exposed to a poxy kid, varicella just comes out of latency as shingles, overcoming the normal barriers a pathogen faces with an otherwise immune population.

The only way to overcome this is to give everyone more chickenpox shots over and over and over again throughout their lives to simulate wild exposure. But even that might not work. Zostavax isn't terribly effective against shingles in general; just against a severe case. But...there's probably a way to make a better vaccine that would more closely simulate "natural" exposure (maybe a live attenuated nasal spray or something).
Even then, we're just looking at keeping the disease at bay for all of eternity. It's still completely beyond even theoretical eradication (unless we have some breakthrough in technology allowing us to devise a vaccine strain that can't go into latency like the current vaccine strains do). But that's science fiction at this point.

So....

[mamakay]

Originally Posted by anewmama

As my daughter says, "Was Dat?"

What % of a population needs to be immune (immune in the "unable to transmit the organism kind of way...not "milder symptoms" like the pertussis vax kind of way) for a disease to be eliminated (local eradication) within that population.

[anewmama]

Has this ever happened entirely due to vaccination? Are claims made about this for smallpox?

[shuttlt]

Wait a minute [narrows eyes] what is this 110% herd immunity threshold? I understand the argument about it being dormant etc... but is the 110% a joke, or does it come from somewhere?

Is there any likelihood of getting shingles from exposure to the vaccine rather than catching chicken pox?

[shuttlt]

Originally Posted by mamakay

What % of a population needs to be immune (immune in the "unable to transmit the organism kind of way...not "milder symptoms" like the pertussis vax kind of way) for a disease to be eliminated (local eradication) within that population.

I had thought that, strictly speaking, the definition was the percentage of the population that needed to be immune for the average number of people each infected person manages to infect is less than one. Clearly with varicella, even if you have herd immunity, it is still problematic to illuminate the illness.

[mamakay]

Originally Posted by shuttlt

Wait a minute [narrows eyes] what is this 110% herd immunity threshold? I understand the argument about it being dormant etc... but is the 110% a joke, or does it come from somewhere?

Is there any likelihood of getting shingles from exposure to the vaccine rather than catching chicken pox?

110% was kind of a joke. But how else do you come up with a figure for a disease that has evolved to overcome herd immunity?

Yes, the vaccine strain can and does go into latency and re-emerge as shingles.

[mamakay]

Originally Posted by shuttlt

I had thought that, strictly speaking, the definition was the percentage of the population that needed to be immune for the average number of people each infected person manages to infect is less than one.

Isn't that a different way of saying the same thing I said?

"The % of the population that needs to be immune for circulation of an organism to "die out" in a community?"

[mamakay]

Originally Posted by anewmama

Has this ever happened entirely due to vaccination? Are claims made about this for smallpox?

It's happened with measles, and probably rubella.

Smallpox, not really. Ring vaccination in combination with quarantine did it, but herd immunity alone didn't ever quite do the trick.

[shuttlt]

Originally Posted by mamakay

Isn't that a different way of saying the same thing I said?

"The % of the population that needs to be immune for circulation of an organism to "die out" in a community?"

I guess I'm being pedantic. You could have a situation where the average number of infections was less than one, but for some reason you kept getting new cases (as has just been described). Now this would still prevent an epidemic though herd immunity, but the illness would never die out.

[mamakay]

Originally Posted by shuttlt

I guess I'm being pedantic. You could have a situation where the average number of infections was less than one, but for some reason you kept getting new cases (as has just been described). Now this would still prevent an epidemic though herd immunity, but the illness would never die out.

I'm pretty sure that it's generally been considered that once there was less than a 1-to-1 level of infection/transmission, that was the mechanism of herd immunity that would initially "slow down" the spread of the pathogen, and eventually lead to elimination.

The language used when discussng "herd effects" is still evolving, though:

http://www.springerlink.com/content/g65222662v6w5h34/

[shuttlt]

Originally Posted by mamakay

I'm pretty sure that it's generally been considered that once there was less than a 1-to-1 level of infection/transmission, that was the mechanism of herd immunity that would initially "slow down" the spread of the pathogen, and eventually lead to elimination.

The language used when discussng "herd effects" is still evolving, though:

http://www.springerlink.com/content/g65222662v6w5h34/

OK. I'll have a read of that. Generally a <1 rate of infection should lead to the pathogen fading away. This is being argued not to be the case with varicella. Ordinarily our two definitions would be equivalent.