http://bmj.com/cgi/content/full/324/7346/1157/a

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BMJ 2002;324:1157 ( 11 May )

Letters

Immunity conferred by smallpox vaccine

    How long does immunity last?
    Authors' reply
 

How long does immunity last?

EDITORI am disappointed that Beeching et al do not provide a reference for their statement that immunity conferred by smallpox vaccination fades after 10-20 years, or earlier in some people.¹ After watching a recent BBC programme about smallpox² I was curious to know whether the smallpox vaccination I had had in childhood would still protect me.

It has been remarkably difficult to find out. Information on the website of the Centers for Disease Control and Prevention says, "Most estimates suggest immunity from the vaccination lasts 3 to 5 years."³ On the other hand, the World Health Organization's website says, "Anyone who has been vaccinated against smallpox . . . will have some level of protection. The vaccination may not still be fully effective, but it is likely to protect you from the worst effects of the disease."⁴

There is little published work on the subject: a Medline search using the search string "smallpox AND immunity AND duration" returned only nine papers, and most of these were not relevant to the question. One of the papers found that immunity can last up to 50 years,⁵ which is in stark contrast to the information on the Centers for Disease Control and Prevention's website.

Instead of saying that immunity fades after 10-20 years, perhaps a more honest statement would be that the duration of immunity is unknown.

Adam Jacobs, director. 
Dianthus Medical Limited, London SW19 3TZ ajacobs@dianthus.co.uk

Authors' reply

EDITORJacobs's letter gives us the opportunity to expand on our statement and to cite the references requested, as there was insufficient space for this in our review. Jacobs highlights the inadequacy of Medline searches when one is dealing with questions that need a historically based answer. He would have found recent correspondence in ProMED to be a more useful source of information (www.promedmail.org/). Hopkins reviewed early writings of Edwardes, which cited higher rates of smallpox in adults than in children during British epidemics in the mid-19th century, reflecting the waning of immunity after unboosted primary childhood vaccination. ^{1 2}

There is considerable evidence that revaccination improves the protection of populations. The Prussians introduced compulsory revaccination every seven years for their army in 1834. This policy resulted in fewer cases of smallpox and deaths in the Prussian army than in the civilian population of Germany or the French army (neither of these was revaccinated) during the Franco-Prussian war of the 1870s.³

In their definitive work on smallpox, which is available on line, Fenner et al tabulate two studies that provide more quantitative modern data.⁴ Hanna reviewed an outbreak of variola major in Liverpool in 1902-3, in which protection against death was substantial after childhood vaccination, even after 50 years.⁵ Mack reviewed 680 cases of variola major imported into Europe and Canada in the mid-20th century; case fatality was 52% in unvaccinated people, 1.4% in those vaccinated 0-10 years before the exposure, and 11% in those vaccinated over 20 years before the exposure.⁶

Tudor and Strati give the risk of smallpox developing in exposed people according to the interval since vaccination, citing 1:1000 at 1 year after vaccination, 1:200 at 3 years, 1:8 at 10 years, and 1:2 at 20 years.⁷ Thus primary vaccination has a clear protective effect against death that lasts for many years. The risk of illness developing after exposure to smallpox 10-20 years after childhood vaccination, however, remains substantial, even if the severity and mortality are reduced. The risk of transmission to others also remains considerable if any illness develops in the index case despite prior vaccination.

We believe that these data support our statement that immunity fades after 10-20 years. In past practice many countries required evidence of revaccination every three years, and during the final stages of the World Health Organization's eradication programme field workers were revaccinated more frequently to provide them with the most solid protection possible.

Nicholas J Beeching, senior lecturer in infectious diseases. 
Division of Tropical Medicine, Liverpool School of Tropical Medicine, Liverpool L3 5QA Nicholas.Beeching@rlbuh-tr.nwest.nhs.uk

David A B Dance, director. 
Public Health Laboratory, Derriford Hospital, Plymouth PL6 8DH

Alastair R O Miller, consultant physician. 
Worcestershire Acute Hospitals NHS Trust, Kidderminster Hospital, Kidderminster DY11 6RJ

Robert C Spencer, consultant microbiologist. 
Bristol Public Health Laboratory, Bristol Royal Infirmary, Bristol BS2 8HW

© BMJ 2002
 

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ALL INFORMATION, DATA, AND MATERIAL CONTAINED, PRESENTED, OR PROVIDED HERE IS FOR GENERAL INFORMATION PURPOSES ONLY AND IS NOT TO BE CONSTRUED AS REFLECTING THE KNOWLEDGE OR OPINIONS OF THE PUBLISHER, AND IS NOT TO BE CONSTRUED OR INTENDED AS PROVIDING MEDICAL OR LEGAL ADVICE.  THE DECISION WHETHER OR NOT TO VACCINATE IS AN IMPORTANT AND COMPLEX ISSUE AND SHOULD BE MADE BY YOU, AND YOU ALONE, IN CONSULTATION WITH YOUR HEALTH CARE PROVIDER.