Sometimes it's easy to forget how common many diseases were before vaccines for them were introduced. When the disease is not around, people may not notice as much.
A recent study attempted to make these "missing" diseases more "visible" by calculating how many were prevented since the introduction of vaccines.
The researchers found that more than 100 million cases of just seven diseases had been prevented since the vaccines were introduced.
This conclusion was based on calculations using data that reached back into the previous century.
More than 20 million cases have been prevented in just the last 10 years, they found.
This study, led by Willem G. van Panhuis, MD, PhD, of the Department of Epidemiology at the University of Pittsburgh, aimed to better understand the patterns of infectious diseases in the US and the effects of vaccination on them.
The researchers first took all the data from 1888 to 2011 that was reported weekly to the Centers for Disease Control and Prevention (CDC) and digitized it.
The data included the cases and deaths from 56 infectious diseases.
Then the authors selected eight specific vaccine-preventable diseases to examine more closely: polio, measles, rubella, mumps, hepatitis A, diphtheria, and pertussis.
For seven of these diseases, the authors estimated the number of cases that have been prevented since the vaccine for that disease was introduced.
They could not calculate this figure for smallpox because the smallpox vaccine was introduced in 1800, before reliable record keeping of cases and deaths was available.
Basically, the researchers subtracted the weekly cases of the diseases in the years after each vaccine was introduced from the total that would have been expected based on historical numbers.
The researchers determined that there were an average 317 measles cases, 28 rubella cases, and 90 mumps cases for every 100,000 people each year before those vaccines were introduced.
The authors noted that measles cases dropped very quickly after the vaccine was introduced, but it took longer for rubella and mumps cases to decline after those vaccines.
The biggest drop for these diseases came after 1978, when the combined measles-mumps-rubella (MMR) was introduced.
Overall, the researchers estimated that approximately 103.1 million cases of these seven contagious diseases have been prevented since 1924 due to vaccines.
However, they estimated that the number could range anywhere from 75 million to 106 million cases, though this is likely underestimated since it only includes numbers from seven vaccine-preventable diseases.
In just the last decade, approximately 26 million cases have been prevented.
"The number of cases that were prevented per disease depended on the incidence rate before vaccination and the duration of the vaccination program," they wrote.
The disease most prevented by the introduction of a vaccine appeared to be diphtheria, which once affected 237 out of every 100,000 people a year.
Since the diphtheria vaccine was introduced in 1924, about 40 million cases have been prevented.
Meanwhile, an estimated 35 million cases of measles have been prevented since that vaccine was introduced in 1963.
Measles was the most common of the seven diseases studied, and its cases declined the fastest after the arrival of the vaccine.
Just five years after the vaccine had been licensed, 95 percent of measles cases that would have been expected each year were prevented.
It took eight years for this to occur for polio, and then 19 years for diphtheria and 17 years for pertussis.
There were some diseases — especially measles, hepatitis A, diphtheria, and pertussis — whose rates varied considerably before vaccines were introduced, probably due to changes in sanitation, hygiene, demographics, or related factors.
"In contrast, the striking and persistent reductions in disease incidence rates after vaccine licensure strongly support the conclusion that vaccination programs were a leading cause," they wrote.
However, the researchers noted that there have still been resurgences of several of these diseases since the vaccines have been introduced, especially measles, mumps, rubella, and pertussis.
The authors attributed these to erosions in herd immunity, in which too few people in a community are vaccinated to prevent the spread of the disease through the population.
Herd immunity can be decreased when a person brings a disease in from a different community, when a person relocates or when individuals in a community refuse or delay vaccination.
"The current low overall incidence of contagious diseases has resulted in a perception that the risk of these diseases is low and, paradoxically, in increased concern about the costs and consequences of vaccination programs," the authors wrote.
This study was published November 28 in the New England Journal of Medicine. The research was funded by awards from the Bill and Melinda Gates Foundation and the US National Institute of General Medical Sciences.
One author has received grants from MedImmune. The other authors reported no financial relationships with industry or other possible conflicts of interest.