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Careful Interpretation of Vaccine Adverse Effects Crucial to Detect Related Events Not Caused By Vaccination

NEW YORK -- October 30, 2009 -- Adverse health events, such as sudden deaths, spontaneous abortions, and Guillain-Barré syndrome, will occur in the general population whether or not they have been vaccinated against influenza A(H1N1), according to a public health paper published online first and in an upcoming edition of The Lancet.

These background rates must be taken account of when such events occur during a vaccination campaign, so that the public does not panic and believe they are caused by vaccination.

Careful consideration of data in this manner will allow authorities to identify when adverse events truly connected to or caused by vaccination have occurred.

Steven Black, MD, Cincinnati Children's Hospital, Cincinnati, Ohio, and colleagues from 13 global medical institutions and health agencies reviewed medical data from prior studies and from hospital databases to identify background rates of health events that occurred without any vaccine. Their review showed the rates of adverse events varied by year, country, and age and sex of the population.

The authors used several examples to highlight their case. On the basis of the reviewed data, if a cohort of 10 million individuals was vaccinated in the UK, around 22 cases of Guillain-Barré syndrome and 6 cases of sudden death would be expected to occur within 6 weeks of vaccination as coincident background cases. There were increased cases of this syndrome during the 1976-77 flu pandemic in the United States. It normally affects about 1 out of every 100,000 people in a year. Based on this background rate, if 100 million people in the US are vaccinated against H1N1, 215 co-incident background cases of Guillian-Barré could be expected to occur within 6 weeks of vaccine -- but they would occur whether or not the vaccine had been given.

The data show that 397 per 1 million vaccinated pregnant women would be predicted to have a spontaneous abortion within 1 day of vaccination. However, this rate would be expected to occur on any day, whether during a vaccination campaign or not. Despite this, it is easy to see how, should clusters of abortions occur during a vaccination campaign, public alarm could be raised despite rates being no higher than these expected background rates.

"Background rates can provide the media, the public, public-health officials, and politicians with important information about the expected number of events that can occur in the absence of any vaccination programme," the authors wrote. "Additionally, they can be used to estimate the number of such events that will occur after immunisation of any number of individuals."

Not taking account of background rates can delay or endanger vaccination campaigns. The authors refer to the 2006 seasonal flu vaccination campaign in Israel, where 4 deaths occurred within 24 hours of vaccination. This was entirely normal, and in fact mortality was lower than expected based on background rates. However, despite these patients being at high risk of death due to underlying medical conditions, their deaths caused widespread panic and global news coverage.

"Misinterpretation of adverse health outcomes that are only temporally related to vaccination will not only threaten the success of the pandemic H1N1 influenza vaccine programme, but also potentially hinder the development of newer vaccines," The authors wrote.

"Therefore, careful interpretation of vaccine safety signals is crucial to detect real reactions to vaccine and to ensure that temporally related events not caused by vaccination do not unjustly affect public opinion of the vaccine. Development and availability of data banks that can provide locally relevant background rates of disease incidence are important to aid assessment of vaccine safety concerns."

SOURCE: The Lancet

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