A virus can only live and reproduce if it has a host to live in. If one person has a cold, the virus can live in that person. When the host gets over the cold, the virus will die and cannot continue to live or reproduce. Its only hope is that the person with the cold will share the virus with someone else. If that person catches a cold, the virus can live on and have chances to find more hosts and spread.
Different viruses have different life cycles. Smallpox, for example, could live on surfaces for a long time. Travelers could pick up smallpox and carry it home in their luggage. Even if they recovered or died, smallpox on their clothing would still have a chance to find a new host.
If someone has a virus and goes to a party with 10 other people, several people may catch the virus. They will become hosts to the virus and give it chance to spread. Each of those 10 people could spread it to 10 more people.
But if the 10 people at the party are vaccinated, chances are good that none of them will catch the virus. None of the partygoers will become a host for the virus, and none will spread it to anyone else.
Imagine that one of the partygoers has a spouse who is not vaccinated. That partygoer probably won’t carry the virus home to their spouse. The unvaccinated person will not catch the virus, because their spouse is vaccinated.
If enough people are vaccinated, the virus will stop spreading. The virus will meet too many vaccinated people and too few unvaccinated people to have a chance to live and reproduce. Even if a few people are unvaccinated, they probably won’t be exposed to the virus.
This is known as “herd immunity.” However, you’ll notice that it doesn’t mean unvaccinated people get immunity. It just means that they are less likely to be exposed. They get to share the protection of their “herd” without being vaccinated.
This is why a few unvaccinated children can stay healthy when most children in a community are vaccinated. Your child’s vaccine against whooping cough protects him, and the community. Most children in the United States today don’t get exposed to whooping cough at all.
On the other hand, lots of us get exposed to the flu every year. And each year brings different variants of the flu. Fewer than half of Americans get a flu shot — in some communities, it’s as few as 16%. At this point, we can’t count on herd immunity to protect us from the flu.
COVID-19 and herd immunity
Different viruses have different characteristics. There’s still a lot we don’t know about COVID-19. We’re fairly confident that it doesn’t live long on surfaces like smallpox. Also, we don’t know what kind of protection people who have had COVID-19 have against catching it again.
Like the flu, COVID-19 will keep developing new strains as it spreads to new people. Every time someone gets the COVID-19 virus, there is a chance a new variant of the virus can occur that is more resistant to the current vaccine.
Only about 30% of Americans are fully vaccinated against COVID-19. This is far too few to offer herd protection to the unvaccinated.
COVID-19 might end up like the flu. Each year, thousands of Americans die from the flu. Many chose not to be vaccinated. Each year, there is a new flu vaccine based on predictions about the variant of the flu that will be in your community. You need a flu shot every year.
We might need a COVID-19 shot, or a COVID-19 booster, each year. If most people choose not to be vaccinated, we will not achieve herd immunity.
Mathematical models suggest that we will reach herd immunity for COVID-19 only when 70% of the population is vaccinated.