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Can vaccines be 100% effective against COVID-19 or any other illnesses?

This article was published on
April 21, 2021

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Some vaccines can be very successful at preventing illnesses or reducing the severity of it, but no vaccine is 100% effective on an entire population. This is because immune responses vary from person to person. The measles vaccine is one of the most successful vaccines, which is 99 per cent effective at preventing the disease. The effectiveness of the flu vaccine administered in the US varies widely from year to year (anywhere from 20% to 60%), depending on how well the annual vaccine attacks that year's mutation of the virus. The duration of how long a vaccine protection lasts, and how many doses might be needed for it to be effective also varies from disease to disease. For the COVID-19 vaccine, the U.S. FDA has set an effectiveness threshold of 50% to be approved or grants the emergency use authorization. A vaccine might not be 100% effective, but if most people get vaccinated then vaccination not only protects individuals, but also prevents the virus from circulating widely in a community.

Some vaccines can be very successful at preventing illnesses or reducing the severity of it, but no vaccine is 100% effective on an entire population. This is because immune responses vary from person to person. The measles vaccine is one of the most successful vaccines, which is 99 per cent effective at preventing the disease. The effectiveness of the flu vaccine administered in the US varies widely from year to year (anywhere from 20% to 60%), depending on how well the annual vaccine attacks that year's mutation of the virus. The duration of how long a vaccine protection lasts, and how many doses might be needed for it to be effective also varies from disease to disease. For the COVID-19 vaccine, the U.S. FDA has set an effectiveness threshold of 50% to be approved or grants the emergency use authorization. A vaccine might not be 100% effective, but if most people get vaccinated then vaccination not only protects individuals, but also prevents the virus from circulating widely in a community.

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What our experts say

Some vaccines can be very successful at preventing illnesses or reducing the severity of it, but no vaccine is 100% effective on an entire population. This is because immune responses vary from person to person.

The measles vaccine is one of the most successful vaccines, which is 99 per cent effective at preventing the disease. The effectiveness of the flu vaccine administered in the US varies widely from year to year (anywhere from 20% to 60%), depending on how well the annual vaccine attacks that year's mutation of the virus. The duration of how long a vaccine protection lasts, and how many doses might be needed for it to be effective also varies from disease to disease.

For the COVID-19 vaccine, the U.S. FDA has set an effectiveness threshold of 50% to be approved or grants the emergency use authorization.

A vaccine might not be 100% effective, but if most people get vaccinated then vaccination not only protects individuals, but also prevents the virus from circulating widely in a community.

Context and background

Vaccine effectiveness studies examine how well a vaccine prevents a particular disease in the “real world,” where people are doing things like going to the grocery store, work, and school. Vaccine effectiveness is a term that is different from vaccine efficacy, because efficacy is used to measure how well a vaccine works to prevent a particular disease in controlled, research environments—like clinical trials.

Preliminary results of clinical trials of COVID-19 vaccines are showing 90-95% efficacy. While public health experts deem this to be very high, some news articles question why the vaccines are not 100% effective.

Resources

  1. Why no vaccine can ever be 100% effective: Wide variation in the population's immune systems mean guaranteed protection is impossible (Daily Mail)
  2. Why aren’t all vaccines 100% effective? (History of Vaccines)
  3. Vaccine Effectiveness: How Well Do the Flu Vaccines Work? (U.S. CDC)
  4. The FDA's cutoff for Covid-19 vaccine effectiveness is 50 percent. What does that mean? (NBC News)
  5. Vaccine efficacy and effectiveness (NZ Immunization Advisory Centre)

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