How to respond to flu vaccine doubters

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In the United States, 3,000 to 6,000 people per year develop Guillain-Barré syndrome, or 1 to 2 of every 100,000, which translates to 80 to 160 cases per week.10 While the exact cause of Guillain-Barré syndrome is unknown, about two-thirds of people have an acute diarrheal or respiratory illness within 3 months before the onset of symptoms. In 1976, the estimated attributable risk of influenza vaccine-related Guillain-Barré syndrome in the US adult population was 1 case per 100,000 in the 6 weeks after vaccination.11 Studies in subsequent influenza seasons have not shown similar findings.12 In fact, one study showed that the risk of developing Guillain-Barré syndrome was 15 times higher after influenza illness than after influenza vaccination.13

Since 5% to 15% of the US population develop symptomatic influenza annually,14 the decision to vaccinate with respect to the risk of Guillain-Barré syndrome should be obvious: vaccinate. The correct question to ask before influenza vaccination should be, “Have you previously developed Guillain-Barré syndrome within 6 weeks after receiving the flu vaccine?” If the answer is yes, the CDC considers this a caution, not a contraindication against receiving the influenza vaccine, since the benefit may still outweigh the risk.


The flu vaccine does not prevent illnesses caused by other viruses or bacteria that can make people sick during flu season. Influenza, the common cold, and streptococcal pharyngitis can have similar symptoms that make it difficult for patients—and, frequently, even healthcare providers—to distinguish between these illnesses with certainty.

One study suggested that influenza vaccine recipients had an increased risk of virologically confirmed noninfluenza respiratory viral infections,15 citing the phenomenon of virus interference that was described in the 1940s16 as a potential explanation. In essence, people protected against influenza by the vaccine may lack temporary nonspecific immunity against other respiratory viruses. However, these findings have not been replicated in subsequent studies.17

Viral gastroenteritis, mistakenly called “stomach flu,” is also not prevented by influenza vaccination.


The prevalence of egg allergy in US children is 0.5% to 2.5%.18 Most outgrow it by school age, but in one-third, the allergy persists into adulthood.

In general, people who can eat lightly cooked eggs (eg, scrambled eggs) without a reaction are unlikely to be allergic. On the other hand, the fact that egg-allergic people may tolerate egg included in baked products does not exclude the possibility of egg allergy. Egg allergy can be confirmed by a consistent medical history of adverse reaction to eggs and egg-containing foods, in addition to skin or blood testing for immunoglobulin E directed against egg proteins.19

Most currently available influenza vaccines are prepared by propagation of virus in embryonated eggs and so may contain trace amounts of egg proteins such as ovalbumin, with the exception of the inactivated quadri­valent recombinant influenza vaccine (Flublok) and the inactivated quadrivalent cell culture-based vaccine (Flucelvax).

The ACIP recommends that persons with a history of urticaria (hives) after exposure to eggs should receive any licensed, recommended influenza vaccine that is otherwise appropriate for their age and health status. Persons who report having angioedema, respiratory distress, lightheadedness, or recurrent vomiting, or who required epinephrine or another emergency medical intervention after exposure to eggs, should receive the influenza vaccine in an inpatient or outpatient medical setting under the supervision of a healthcare provider who is able to recognize and manage severe allergic reactions.

A history of severe allergic reaction such as anaphylaxis to a previous dose of any influenza vaccine, regardless of the vaccine component (including eggs) suspected of being responsible for the reaction, is a contraindication to influenza vaccination. The ACIP recommends that vaccine providers consider observing patients for 15 minutes after administration of any vaccine (regardless of history of egg allergy) to decrease the risk of injury should syncope occur.20


Table 3. Two different forms of mercury
Thimerosal is a preservative that has been used to prevent the growth of bacteria and fungi in multidose vials of vaccines and medicines in the United States for several decades.21 It is important to understand the difference between ethylmercury in thimerosal and methyl­mercury found in certain foods (Table 3), as ethylmercury is safe, but methylmercury may not be safe, particularly when ingested in large quantities.

A process of biomagnification of methylmercury occurs when humans eat large fish that have eaten smaller fish. Thus, larger fish such as shark can be hazardous for women who are or may become pregnant, for nursing mothers, and for young children, while smaller fish such as herring are relatively safe.

As a precautionary measure, thimerosal was taken out of childhood vaccines in the United States in 2001. Thimerosal-free influenza vaccine formulations include the nasal live-attenuated flu vaccine, the inactivated quadrivalent recombinant influenza vaccine, and the inactivated quadrivalent cell culture-based vaccine.


At least 10% of US adults have aichmophobia, the fear of sharp objects including needles.22 Vasovagal syncope is the most common manifestation. Behavioral therapy, topical anesthetics, and systemic anxiolytics have variable efficacy in treating needle phobia. For those who are absolutely averse to needles, the nasal flu vaccine is an appropriate alternative.


Some immunosuppressive medications may decrease influenza vaccine immunogenicity. Concomitant administration of the inactivated influenza vaccine with other vaccines is safe and does not alter immunogenicity of other vaccines.1 The live-attenuated influenza vaccine is contraindicated in children and adolescents taking aspirin or other salicylates due to the risk of Reye syndrome.

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