Effectiveness of Flu Vaccine Raises More Red Flags

By: Patrice La Vigne

The news may  be well known now about the influenza vaccine being only 56 percent effective  overall and 9 percent effective for the type A H3N2 strain in adults aged 65  years or older during the 2012-13 flu season. ¹ The latest buzz, however, is about how getting a flu shot year after year may make  people more susceptible to getting  influenza.

Year-to-Year Flu Vaccination Could  Limit Effectiveness

In February, Clinical Infectious Diseases published a  new prospective study noting the failure of the flu vaccine in people vaccinated  against influenza during the previous year.

The  researchers followed 328 households with 1,441 members from before the  2010-2011 flu season through the end of it. A total of 866 study participants  received the flu shot before the flu season started. Nearly one quarter of the  households with 125 members contracted the flu during the 2010-11 season, as  confirmed by laboratory tests.

When the  researchers separated out those, who were not vaccinated in the previous flu  season, they found less of a benefit with the flu vaccine. The influenza  vaccine was 62 percent effective among people, who did not receive a flu shot in  the prior year. In comparison, vaccine effectiveness among those, who did get a  flu shot in the previous year, was substantially lower at -45 percent.

Furthermore,  the study found that those who were vaccinated in both years and those who were  not vaccinated in either year had similar influenza infection risks. ²

Seasonal Flu Vaccine Could Increase  Future H1N1 Infection Risk

This is not  the first study questioning vaccine effectiveness based on repeated annual  influenza vaccination. In fact, there is a large body of literature examining  the impact of previous use of seasonal trivalent flu vaccines on the risk of contracting  the 2009 pandemic H1N1 swine flu.

In 2010, PLoS Medicine published an analysis of  Canadian epidemiological studies suggesting that people, who had received seasonal  flu shots the year before the 2009 H1N1 swine flu pandemic, had an increased  risk of becoming infected with the pandemic swine flu.

During the  spring 2009 pandemic H1N1 virus wave, investigators noted a link between prior  receipt of the trivalent inactivated influenza vaccine during the 2008-09 season  and fever/cough illness. In response, they conducted four follow-up  epidemiologic studies (three case-control studies and one household  transmission study) during the summer of 2009 to further explore the original  finding.

Studies Identify Flu Vaccine Failures

Findings from  one of the case-control studies showed that prior vaccination with trivalent  inactivated influenza vaccine is protective against seasonal influenza and  reduced the flu risk by 56 percent. However, results from all four studies  revealed that those vaccinated in the previous 2008-09 season were between 1.4  and 2.5 times more likely to contract H1N1 during the spring and summer of  2009. ³

A second  study in 2009 identified a similar association between previous vaccination and  pandemic H1N1 illness in a military population. Between April 21 and May 8, a  total of 97 patients developed the H1N1 virus. Of these, 63 people, or 66  percent, received the influenza vaccination in the previous 12 months. In  comparison, only 40 percent of patients without H1N1 virus had no history of  vaccination. ⁴

In Nov. 13,  2009, the Center for Disease Control and Prevention published a case-cohort  study on the effectiveness of 2008-09 trivalent influenza vaccine against 2009  pandemic H1N1. The CDC concluded that there was no decreased or increased risk  of pandemic H1N1 following seasonal flu vaccine. ⁵

Doctors Intrigued and Troubled by  Findings

In an  accompanying editorial, Drs. John Treanor and Peter Szilagyi wrote a response  to the findings for the most recent study showing lower effectiveness with  back-to-back flu shots.

“As we are  currently struggling through one of the most vigorous influenza seasons in  recent memory, the apparent failure of influenza vaccine under optimal  conditions seen in this study is indeed troubling,” they wrote. ⁶

As usual,  further study on flu vaccine effectiveness is necessary. There are a lot of  conflicting data available. However, the growing number of studies showing low  overall effectiveness, waning immunity and a negative effect of prior-year  vaccination cast doubt on influenza vaccine policies and strategies, especially  when influenza vaccinations now are being required as a condition of employment  for health care workers.

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