We call them the diseases of childhood - measles, mumps, rubella and chickenpox, to name a few.
But now that these diseases seldom circulate in countries that immunize against them and immune protection is rarely being naturally refreshed or "boosted" by sporadic exposure, is there a risk that in the future, older adults may find themselves unexpectedly vulnerable to these disease pests from the past?
As we head into a world where an ever-growing - and aging - proportion of the population only has vaccine-acquired protection, what is really known about how long immunity is likely to endure? For that matter, can science be sure that immunity generated by infection - thought for some of these diseases to be lifelong - will endure into old age, in the era of vaccines?
"I don't think we know much at all," acknowledges Samuel Katz, co-inventor of the measles vaccine and a pediatric infectious disease expert at Duke University in Durham, N.C.
Mimi Meade, 7, receives one of the first U.S. polio vaccine injections, in 1954. Experts don't know how long other childhood immunizations last. (Harvey Georges/AP)
Figuring out answers about the durability of immunity - naturally and vaccine-acquired - in a time without natural boosting won't be easy.
But the last generation to have routinely suffered through most of these diseases is crossing through mid-life and the first generation to have avoided them is hovering around 40.
As both groups head toward the golden years, when any waning of their disease defences may be exacerbated by the age-related decline of the immune system, gauging the levels of society's immunity could become key to keeping these nasty invaders out of our communities, experts say.
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," says Dr. Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
Before vaccination became commonplace, adults often came in contact with youngsters suffering from mumps, measles and the other childhood diseases. That remained the case in the early days of vaccine administration when these diseases still commonly circulated.
If people had protection - natural or vaccine-acquired - those exposures were actually helpful. They acted as a sort of natural booster shot, reminding the immune system to be on guard for this threat.
Some experts now wonder whether these unrecorded natural boosts may have led the medical community to overestimate the durability of immunity generated by childhood vaccinations. These days, few people are getting natural boosting to these diseases.
Questions about the durability of immunity are on the minds of public health authorities in countries where childhood vaccines have been in use the longest, says Jane Seward, an expert in vaccine-preventable diseases with the U.S. Centers for Disease Control in Atlanta.
"It's certainly a reasonable hypothesis that immunity might wane more quickly in the absence of external boosting. Whether that's the case or not, we don't know. But it's a reasonable thing to postulate," she says.
Dr. Seward's group at the CDC is following people who were vaccinated with the measles-mumps-rubella vaccine about 15 years ago to track how well their protection is holding up. They hope to mount a similar effort to study the chickenpox vaccine, which was put into broad use in the United States and Canada in the mid- to late-1990s.
Others, though, acknowledge that the long-term studies needed to assess immunity levels generated by the range of childhood vaccines have not been systematically undertaken.
"We have not methodically - we being the field of public health officials, scientists - we have not methodically measured the level of immune responses to standard childhood diseases vaccines that people have received one, two, three, four, five decades earlier," says Anthony Fauci, director of the U.S. National Institute of Allergy and Infectious Diseases.
"It has been done mostly in a reactive mode," he says, pointing as an example to work done to understand a large outbreak of mumps in Iowa in 2006.
The full picture of the durability of immunity in the vaccine age will probably take decades to come into sharp focus. And the answers will probably vary from disease to disease, Dr. Fauci and Dr. Katz say.
The fact that protection may not be lifelong should not be characterized as a failure of the vaccines, public health experts say. The years of protection they have already conferred have dramatically slashed rates of once-common diseases.
Consider measles: While 300,000 to 400,000 Canadian children used to contract the disease every year, now an outbreak of a handful of cases - such as a recent cluster in Toronto - makes the news.
Given that measles has a complication rate of 20 per cent and that the World Health Organization estimates 242,000 children died from the disease in 2006, proponents hail vaccines as a modern-day medical miracle.
Experts also warn that the fact immunity may not be lifelong should not be used to argue for natural infection over immunization.
"Never forget natural infection comes at a great cost, both potentially to the individual and definitely to society," Dr. Osterholm says. "Each infection is a crapshoot as to whether it's going to be mild, moderate, severe or fatal."
The end result of the investigation into the durability of immunity in the vaccine age could be a recognition that adults need booster shots to prevent outbreaks of what we now consider childhood diseases.
If emerging data show adults will need booster shots for childhood diseases, vaccine delivery programs will have to be reorganized to look at issues such as payment and how best to ensure adults actually get the shots, says Bonnie Henry, chairwoman of the Canadian Coalition for Immunization Awareness and Promotion.
"Adult immunization is woefully neglected," notes Dr. Henry, an infectious-diseases expert with the British Columbia Centre for Disease Control.
"And I don't know what the answer's going to be in that, but it does pose a whole bunch of challenges because we don't have the access to [adults] in the same way that we do when you're in school."
