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The 6 month protection as a rule of thumb is from an era when girls actually
had chicken pox. Even then, the 6 month time estimate was an dichotomous
approximation of a probability curve. A few babies would have enough
antibodies to still inactivate the chicken pox vaccine up until 12 months of
age. A tiny minority were at risk before 6 months.

Measles is better studied. The measles vaccine is given in those rare
situations when infant between 6 and 12 months old is exposed, but that shot
doesn't count and is repeated at 12 months of age under the assumption that
it might have been deactivated. Some 12 months old infants will still have
maternal antibodies, and the MMR was originally given at 15 months of age.
When we went to the two shot MMR series, at 12 months and 4 years old, the
gaming strategy changed. Giving the shot at 12 months of age exchanged a
slightly higher risk of vaccine failure at that age for the benefits of
protecting the larger number of 12-15 month old children who had already
lost their antibodies. The higher risk of failure was ameliorated with the
second dose at 4 years old.

For chicken pox, the vaccine was given as a one shot starting in 1995 in the
U.S. Japan started in 1988, if my memory is correct, but its use was not
widespread and the disease remained pandemic so that the vaccinees were
still exposed to the wild virus acting as a booster. In 2007, the U.S.
recognized that antibody levels were decreasing in adolescents and a 2nd
Varivax shot was added. Perhaps without occasional exposure to the wild
virus to act as a booster, one shot was not enough to establish lifelong
immunity. (I had been warning my primary care patients for several years
that I expected the booster would be necessary.) I would hypothesize we will
be seeing many women having children in the next decade who only had one
shot. Their low antibody levels may be adequate to protect the vast majority
of them, but may not be plentiful enough to supply protection to their
babies. Time will tell.

Kevin Powell MD PhD FAAP
Saint Louis, MO

-----Original Message-----
From: Pediatric Emergency Medicine Discussion List
[mailto:[log in to unmask]]On Behalf Of JaPe
Sent: Thursday, August 27, 2009 8:37 PM
To: [log in to unmask]
Subject: Re: Varicella


Julia,
For the reason you mention, how did you confirm the diagnosis of varicella?
Was it on clinical grounds or serology? Conventional wisdom is that maternal
antibodies (if present) are generally protective until 6 months or so..
Thanks in advance
Jay
Jay Pershad
Le Bonheur

-----Original Message-----
We recently saw a 3 month old who not only had chickenpox but also a
superinfection. Do any of you have much experience with this age group and
chickenpox, perhaps a similar case and do you have a good reference article?
Thank you!
Julia Whitefield MD, Ph.D.

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