Dear Dr. Roach: I was wondering if you could explain why some vaccinations seem to completely protect against a disease, while others do not. For example, smallpox was completely eradicated by vaccination, and the hepatitis B vaccine is good for life and 98% to 100% effective, according to the World Health Organization. Yet we need to have the flu vaccine each year. I understand that is partly due to different strains of flu that come through, and of course we have the coronavirus vaccine, which … I hope you can see where this question is going. I think an understanding of the vaccines we have and how they work would be extremely beneficial to all.
M.S.
It is hard even for experts to predict the effectiveness of a vaccine, and these experts have been pleasantly surprised at how effective some vaccines were and disappointed at others.
The smallpox vaccine was about 95% effective at preventing a person from getting a case, but its effectiveness started to wane after three to five years. Smallpox was eradicated through multiple means, not just vaccination. Quarantine remained a critical tool. When a case was identified, everyone who was in possible contact with that person was vaccinated or re-vaccinated (a strategy called “ring vaccination”).
The global eradication of smallpox was an immense effort, requiring not only enormous resources but also the political power to ensure its success. It was often met with resistance by anti-vaccine activists. They became vocal within a few years of the creation of the vaccine in 1796 and used some of the same misinformation that is still being spread today.
However, mortality rates of 15% to 30% led to mandates that were enforced in the U.K. and the U.S., which have been repeatedly upheld by the U.S. Supreme Court. These mandates led directly to dramatic drops in smallpox deaths.
Vaccines against the influenza and coronaviruses are hampered by the virus’ ability to mutate and escape the immune system of people who have been previously infected or vaccinated, in addition to the waning of immunity over time. Although the measles virus mutates, it has not mutated much. The vaccines made in the 1960s are still highly effective.
One other important factor in the eradication of infectious diseases is the concept of herd immunity. People who have been successfully vaccinated or who have survived infection from a viral disease often cannot transmit it to others, leading to a smaller and smaller population at risk for the disease. The current COVID vaccines make transmission less likely, but not impossible. This means a person who is unvaccinated, or in whom the vaccine has been unsuccessful, is less likely to be exposed. That’s why not everyone needs to be vaccinated in order to eradicate a disease, but the more people who are, the faster the disease can be eradicated.
Hepatitis B and the human papillomavirus (HPV) strains that cause cancer have the potential for eradication, and there is evidence this can be achieved. Still, smallpox remains the only human disease eradicated.
The global nature of travel brings vast new challenges with the coronavirus pandemic. Areas where viral transmission remains high will lead to new strains, with the potential for catastrophe due to more dangerous mutations, which is why mass vaccination of COVID-19 is a global priority. It remains to be seen whether there is the political will to take the steps to do so.
Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to [email protected]
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