Effectiveness of vaccine against pandemic influenza A/H1N1 among people with underlying chronic diseases: cohort study, Denmark, 2009-10

BMJ 2012; 344 doi: http://dx.doi.org/10.1136/bmj.d7901 (Published 25 January 2012)
Cite this as: BMJ 2012;344:d7901

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Whilst this is a well conducted study, its ability to provide a genuine estimate of protection is hampered by the fact that the vaccine was only made available very late in the epidemic. Many of those who were vaccinated were likely to be incubating influenza or were likely to be exposed and develop clinical illness before any protective effect kicked in (which takes 2 weeks, as was explained in the article). It is also feasible that many patients may also have actively sought vaccination because they had already started to feel unwell, generating an “ill patient” vaccination bias.

Pandemic H1N1vaccination was 49% effective 14 days after administration, which demonstrates it still conferred what I would regard as clinically relevant protection. It is good to know that timely vaccination will stop half of our vulnerable patients developing clinical influenza. The additional finding in this study that there was a greatly increased risk of influenza occurring in those with chronic comorbid illnesses indicates the value of continuing to target this group in vaccination campaigns.

As to the "legal consequences", I think one should bear in mind that when there is a vaccine with proven 49% - 71% efficacy against a life-threatening infection, the legal consequences of failing to offer it to vulnerable patients who are demonstrably at greater risk might be more significant.

Competing interests: None declared

Peter J Flegg, Physician

Victoria Hospital, Blackpool FY3 8NR

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Why so little comment about the Danish study of flu vaccine effectiveness in people with chronic diseases? At best the pandemic vaccine was only 49% effective in preventing lab-confirmed disease from pH1N1 after 14 days, and in the first week after vaccination it nearly quadrupled the associated risk of being hospitalized with pH1N1! Furthermore, prior receipt of seasonal flu vaccine alone nearly tripled the associated risk of hospitalization with pH1N1!

This was a large study in an integrated health system that efficiently linked vaccination with outcomes and effectively overcame selection bias. It should prompt us to reconsider national flu vaccination programs, but no one is talking.

Were the authors' conclusions too understated? Have we been distracted by the laboratory bird flu flap? Has the current flu season thus far been mild enough to quiet our passions?

Competing interests: None declared

Allan S. Cunningham, Retired Pediatrician

Cooperstown, NY--USA, Cooperstown, NY

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Dear Editors,

This detailed study provides solid evidence for not vaccinating high risk patients against influenza H1N1.

One thing is developing and measuring antibodies against influenza virus H1N1, and another thing is to actually obtain clinical benefit after vaccination.

Hundreds of thousands were vaccinated in this Danish study, but hospital admissions due to H1N1 influenza virus did not diminish!

There results come with some interesting legal consequences: no one can actually declare anymore that after vaccination the patient’s overall health is protected.

NHS Trusts, together with other European Health Ministries, will find it extremely difficult to dedicate hundreds of millions of pounds in extended vaccination programs that do not reduce hospitalizations of high risk patients!

Competing interests: None declared

Stavros Saripanidis, Consultant in Obstetrics and Gynaecology

Private Surgery, Thessaloniki, Greece

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