Repeated flu shots may blunt effectiveness


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niversal influenza immunization for participants who received both seavaccine effectiveness in those who’d programs, available in virtually sons’ vaccines. had flu vaccine over the previous five every province and territory, may Those are not statistically significant years, compared to those who’d had need to be reconsidered in light of emergfindings, “but when you’re dealing with only the previous year’s vaccine or no ing evidence that repeated flu shots may a low vaccine effectiveness, even 40%, vaccine previously. blunt the vaccine’s effectiveness in subseand a null effect in those who received The findings are also consistent with quent seasons. vaccine the previous year, you need a a 1999 observation of variable flu vacThat phenomenon was seen cine efficacy depending on in the Jan. 29, 2015 interim immunization history, by estimates of the effectiveness Derek J. Smith, the current of the 2014/15 vaccine against director, WHO Collaborating influenza A (H3N2) from CanCentre for Modelling, Evoluada’s Sentinel Physician Surtion and Control of Emerging veillance Network, headed by Infectious Diseases. Smith proDr. Danuta Skowronski of the poses “antigenic distance” as British Columbia Centre for an explanation. This hypotheDisease Control. sis holds that the impact on the The effects of repeat immucurrent season’s vaccine effecnizations need to be studied tiveness is most pronounced further. Meanwhile, a return to when the difference between targeted, high-risk flu vaccine Research in both Canada and the US indicates that getting the flu last season’s vaccine compoprograms, rather than universal shot several years in a row may lessen the vaccine’s effectiveness. nent and the current season’s coverage, seems warranted, vaccine component is small said Skowronski, the BC centre’s epidemassive sample size to prove that statis(this year they’re identical). miology lead for influenza and emerging tically,” Skowronski said. “If we wait In that case, the current season’s vacrespiratory pathogens. for that we’ll never be able to tease apart cine may boost memory of the earlier At the same time, antivirals — forthese signals.” vaccine’s antibody responses; then these merly plan B in the primary prevention Dr. Richard Schabas, Ontario’s for“old” antibodies may mop up the current of influenza — should be used early. mer chief medical officer who is now vaccine’s antigen before the newly vac“There’s a lot of debate about antivirals medical officer of health in the Hastings cinated person can mount an immune and that needs to be resolved also, but and Prince Edward Counties Health Unit response to it (this is called antibody for now, [early antiviral use] should in Belleville, Ontario, agrees that it’s interference). come to the fore,” Skowronski said. time to revisit universal flu vaccine proFor now, researchers need to invesThe BC network’s estimates of this grams, which are now available in all tigate the phenomenon, Skowronski year’s flu vaccine efficacy, published in Canadian provinces and territories said. And public health officials need to Eurosurveillance, were –8% overall and except BC, Quebec and New Brunswick. reconsider the wisdom of targeted flu 2% in young adults against medically He was once a proponent of univervaccine programs, especially in a year attended, laboratory-confirmed influenza sal programs but now says, “We should when the elderly have been disproporA (H3N2) infection — which Skowronuse the next six months or so to do a tionately affected. ski said she interprets as a null effect. very careful rethink of where we are. On Feb. 5, 2015, other Canadian This also represents the lowest measured There’s enough new evidence that we researchers published interim estiprotection against a seasonal virus in the should all be troubled enough by, that mates of 2014/15 influenza vaccine program’s 10-year history, she added. we should be taking a long and sober effectiveness in preventing laboratoryIt recently became known that this look at our policies. There are more and confirmed influenza-related hospitalyear’s H3N2 virus was not a match for more unanswered questions about how ization. The figure for people aged 65 the H3N2 component of this year’s trieffective a universal program really is.” and over was –32.9%, according to valent vaccine, but the authors of this Canada’s Sentinel Physician Surfindings from the Serious Outcomes new study also saw variability in vacveillance Network has previously Surveillance Network of the Canadian cine effectiveness that was related to observed this decrease in effectiveness, Immunization Research Network. — prior vaccination history. Vaccine effecas have others. In the September 2014 Terry Murray, Toronto, Ont. tiveness was 43% for those who hadn’t issue of Clinical Infectious Diseases, received the 2013/14 vaccine but –15% US investigators documented a lower CMAJ 2015. DOI:10.1503/cmaj.109-5000 E180

CMAJ, April 7, 2015, 187(6)

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Repeated flu shots may blunt effectiveness.

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