We seem to be doing a lot of vaccination stuff here lately. It’s an obvious public health topic, one that’s in the news and (in some quarters) considered controversial. I’m a strong proponent of vaccination where it makes sense (which is in most of the instances where it is used) but that doesn’t mean I think it is problem free. For a public health scientist the problems are not only interesting but of practical import. Yesterday’s post about fainting during vaccinations produced an unexpected comment thread from people who have at one time or another fainted during a vaccination or medical procedure, for example. The rare adverse event, even if it is one in a million or ten million, can affect public confidence in a whole vaccination program since when you are giving tens of millions of doses, even rare events are bound to occur, even though the net benefit to the public is very large compared to the tiny risk. But one issue that has been hidden, probably because many of our most effective and important vaccines are relatively recent, is our ignorance about how long they are effective. This arose recently in a multi-state mumps outbreak where it turns out most of the indigenous cases were among those previously vaccinated against the disease. Apparently the vaccine’s effect had waned. This has raised some interesting questions.
When a disease circulates in a community, adults who were immune as a result of childhood infection receive continual “booster infections” that keep their immunity current and “topped off.” When a substantial portion of the population is vaccinated with a effective vaccine, however, the virus no longer circulates in the community and the booster effect doesn’t occur. If there is a natural waning of immune protection in the absence of new exposure to the virus we could produce an aging population with little or no protection to some important childhood diseases. If the world’s population were as well covered by vaccines as in the US, this wouldn’t be as much of a problem because the virus wouldn’t be circulating anywhere. But that’s not the case and the mumps outbreak started from unvaccinated cases that acquired natural infection outside the US and were then able to infect US cases whose vaccine-acquired immune protection had decayed.
The remedy for this might be to give booster shots to adults at an appropriate interval — if we knew what an appropriate interval was:
“I don’t think we know much at all,” acknowledges Dr. Samuel Katz, co-inventor of the measles vaccine and a pediatric infectious disease expert at Duke University in Durham, N.C.
Figuring out answers about the durability of immunity – naturally and vaccine-acquired – in a time without natural boosting won’t be easy.
Dr. Michael Osterholm says scientists should be doing long-term immunity studies – following groups of people for decades – in the way cancer researchers track groups of people to try to discern what causes cancer.
“That would help us understand at what point does the level of protection drop for a population. Not any one individual. But a population norm where you would now recommend that a booster shot should occur as a standard of medical practice,” suggests Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota. (Helen Branswell, Canadian Press)
There are a bunch of difficult questions here. Is there a way to know, say by measuring a particular antibody level, when a person’s immunity has sunk to a level it is no longer protective? This is a scientific question that goes beyond childhood vaccines. For example, in testing proposed vaccines against influenza A/H5N1 (aka bird flu) we don’t know if they protect people because (fortunately) we haven’t had an outbreak of transmissible bird flu. As a fall back we measure the level of neutralizing antibodies in the blood of vaccinated subjects and, based on previous experience with other kinds of flu, guess at a protective level. It may be a pretty good guess, but it’s still a guess. We have similar problems in following the immune status of previously people vaccinated against other diseases long ago. Like mumps.
This is very tough science and just one of many urgent public health research questions that affect all of us. What’s the Bush administration response?