Thursday, November 13, 2014

Influenza vaccine recommendations: Stop needling me!

File:14234CDC Flumist.tifSeasonal influenza is responsible for an estimated 200,000 hospitalizations and 23,000 deaths in the US annually. Each year influenza vaccines are produced based on the viruses forecast to become prevalent. There are two types of vaccine: inactivated influenza vaccine (IIV), delivered via injection, and live attenuated influenza vaccine (LAIV), delivered via a mist sprayed into the nose. Influenza vaccines typically have efficacies exceeding 60% and an estimated 46% of the American public relieves vaccine annually.

While many people are vaccinated each year, it is desirable to increase vaccination rates for at least two reasons. First, vaccine-associated immunity protects individuals from developing potentially serious or fatal disease. Second, high population coverage produces a herd immunity effect: those possessing vaccine-associated immunity cannot become infected and thus cannot infect others. This is especially important for protecting individuals for whom vaccines are contraindicated.

Individuals who are immunocompromised or immunosuppressed are such a group. Consider, for example, patients recovering from hematopoietic stem cell transplantation (HSCT) following myeloablative conditioning. In cases of imperfect donor-recipient match, patients may take immunosuppressive medications as prophylaxis against, or treatment for, graft versus host disease. During this process of immunologic tolerization, which can last months or longer, patients must avoid crowds and limit work/school and social interactions in order to avoid potentially fatal infections. And during this period it is critically important for caregivers and contacts to be vaccinated against influenza and other vaccine-preventable diseases so that they do not become infectious.

LAIV is contraindicated for caregivers of such persons in the Advisory Committee on Immunization Practices (ACIP) guidelines. Because LAIV contains live influenza viruses, a potential exists for transmission of vaccine strain viruses from vaccinees to other persons. The period of viral shedding in vaccinees is variable and relatively short lived. Vaccinated immunocompetent children, for example, shed vaccine viruses for less than 3 weeks, and evidence suggests that shedding may be much shorter lived than that. LAIV-associated shedding occurs in lower titers than is typically observed in disease-associated shedding caused by wild-type influenza viruses.

As several studies have demonstrated higher efficacy of LAIV relative to IIV in children (but see the footnote below), the ACIP has expressed
a preference for the use, when immediately available, of live attenuated influenza vaccine (LAIV) for healthy children aged 2 through 8 years, to be implemented as feasible for the 2014–15 season but not later than the 2015–16 season.
Higher protective efficacy of LAIV in children provides strong rationale for the ACIP statement. Moreover, promoting LAIV as an alternative to IIV in older patient populations may result in increased coverage in those who avoid vaccination due to fear of needles. I wonder if increased use of LAIV might pose additional risk to immunocompromised persons, however, in terms of inadvertent exposure to recent vaccinees shedding live, though attenuated, influenza viruses. Such patients may need to become more meticulous in screening visitors and contacts who may have received LAIV recently.

Footnote: During 2013-2014 there was no measurable effectiveness for LAIV against influenza A (H1N1) among children enrolled in effectiveness studies. The reasons for this are unclear.

(image source: Wikipedia)

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