Ask Stanford Med: Dekker answers flu vaccine queries

Cornelia Dekker

While the 2011 influenza season was especially mild, that may not be the case this year. To help people prepare for the flu season, Cornelia Dekker, MD, medical director of the Stanford-Lucile Packard Children's Hospital Vaccine Program, responded to questions about the flu and vaccine research in the "Ask Stanford Medicine" feature on the medical school's blog,Scope.

The following Q&A is adapted from questions submitted to Dekker via the @SUMedicine Twitter feed and Scope, ranging from the effectiveness of the flu shot to concerns related to vaccinating children. The complete version is available at http://scopeblog.stanford.edu/category/askstanfordmed/. Information about clinical trials for Stanford's vaccine programs are available at http://vaccines.stanford.edu/clinical_trials.html.

Q: What percentage of patients receiving the injection still contract the virus? Additionally, what is the selection process for choosing which strains of the virus will be contained in the vaccine from year to year?

Dekker: The definition of influenza vaccine effectiveness is complicated by many considerations, but the general answer is that it provides 70-90 percent protection overall. Keep in mind that influenza vaccine composition is redefined every year. Some years this results in a vaccine with a good match to the viruses that circulate during flu season. However, during other years the vaccine is not so well-matched against one or more of the three influenza strains chosen. This and other factors contribute to variation in the published rates of vaccine effectiveness.

Every year, viruses are chosen to represent influenza A/H3N2, A/H1N1 and influenza B for the upcoming year's flu vaccine. Selection of the virus strains is made in late February by the World Health Organization after detailed analysis of influenza viruses collected by more than 100 national centers during the prior year's flu seasons. A prediction is made for the next season based on this analysis and availability of new candidate virus strains is confirmed. After the WHO makes its general recommendation, each country determines which exact virus strains will be used in its licensed vaccines. For the United States, the group deciding this is the federal Food and Drug Administration in consultation with its advisory committee of vaccine experts.

Q: A recent article in Scientific American reports there is a lack of scientific evidence that flu vaccines are effective for the elderly or very young. What is your perspective on this research?

Dekker: I think it confirms that while there is certainly room for improved influenza vaccines for individuals at the extremes of age and for those with some medical conditions associated with poor vaccine response, there is still value to getting immunized. Importantly, we can't predict at the individual level who will be protected and who will still become sick from influenza infection. This was borne out in a recent study from the U.S. Centers for Disease Control and Prevention that indicated 43 percent of 817 children who died of influenza over the eight influenza seasons in the United States had no predisposing illness and the time from symptom onset to death was in fact shorter among children with no underlying high-risk conditions compared with children with at least one high-risk condition. This reinforces our current recommendation to vaccinate all children annually against influenza if older than 6 months of age, with very rare exceptions.

For the elderly, the only trial that compared vaccinated vs. unvaccinated individuals using a randomized, controlled design was done in the Netherlands during the 1991-92 influenza season. Vaccine efficacy was 58 percent for preventing clinically defined influenza with serologic confirmation of infection in this study. Because we now recommend that everyone get a flu shot, it is no longer possible to do such studies with unvaccinated controls.

Q: I've read that getting a flu shot during pregnancy can lower the risk of babies being born prematurely or underweight, as well as boost their immunity. Can you explain how getting a flu shot while pregnant can result in such added health benefits for newborns?

Dekker: These results come from a randomized, controlled trial conducted in Bangladesh of influenza vaccine vs. pneumococcal vaccine given to pregnant women and a retrospective cohort analysis of a large surveillance data set (the Georgia Pregnancy Risk Assessment Monitoring System) done by the same authors. In both studies, maternal influenza immunization was associated with higher birth weight and lower risk of premature birth — but only during periods when influenza virus was circulating in the community. This implies that the effect is from prevention of influenza infection in the mother, infection that itself is associated with premature labor and lower birth weight infants. The authors speculate that the vaccine effect may be mediated by preventing release of inflammatory cytokines associated with preterm labor and from stimulation of prostaglandins that provoke uterine contraction. The effect on birth weight was more prominent in the Bangladesh cohort and may also be explained by inflammatory mediators having an effect on maternal and infant metabolism. More work needs to be done to follow up on these intriguing findings.

Q: Is there a vaccine for H3N2?

Dekker: An influenza A/H3N2 virus is included as part of each annual influenza vaccine "cocktail." This year, the following three vaccine viruses are included in the 2010-13 influenza vaccine formulations:

• A/California/7/2009 (H1N1) pdm09-like virus. This is the same virus used since the 2009 pandemic.
• A/Victoria/361/2011 (H3N2)-like virus. This is a change from last year.
• B/Wisconsin/1/2010-like virus. This is from the B/Yamagata lineage of viruses, which is also a change from last year.

If a new circulating virus strain with pandemic potential is identified after manufacturers have already begun manufacture of the fall influenza vaccine, it is possible to make a matched pandemic vaccine as was done in 2009. Newer methods of vaccine manufacture will shorten the manufacturing timeline in the future.

Q: Does the flu shot pose any specific risks for children with autoimmune disorders or diseases? Also, is a preservative-free shot available?

Dekker: It's important that children with autoimmune disorders get immunized annually because they may be at higher risk of getting serious complications of influenza disease. Children with the following chronic health problems are at high risk (for most diseases on this list, the recommendation is to get the shot rather than the nasal spray version of vaccine that contains live but weakened virus, so check with your doctor):

• Asthma
• Neurological and neurodevelopmental conditions
• Chronic lung disease
• Heart disease
• Blood disorders
• Endocrine disorders
• Kidney disorders
• Liver disorders
• Metabolic disorders
• Weakened immune system
• Those on long-term aspirin therapy

Preservative-free influenza vaccines are available as single-dose injections so just ask your provider if you have a concern. The nasal spray vaccine does not contain preservative.

Q: I am an RN and required to get a flu shot every year for my job, but my husband has never had a flu shot. We have an infant son with heart problems. Is it important for my husband to get a flu shot annually because of the baby, or can he skip it?

Dekker: It is important for your husband to get the flu vaccine to maximize protection for your son and, of course, also himself. Remember that the first time a child gets the flu vaccine it takes two doses a month apart to fully immunize. Children and adults with heart disease are in one of the high-risk groups for influenza complications; more information about children and influenza can be found on the CDC website.

Depending on his age, your husband probably has the choice of three vaccine formulations: the traditional flu shot, the nasal spray vaccine or the new microneedle flu shot.