A: Some diseases are more elusive than others when it comes to herd immunity, and your two examples are among the elusive ones.
In order to be a candidate for herd immunity, we need a disease caused by one specific virus. The virus has to be easily recognized by the immune system; it can’t mutate particularly rapidly; and it has to be transmitted in a process that only involves humans–not mosquitos or birds or rusty nails. COVID-19 meets all of these benchmarks, so we still think that herd immunity is achievable–eventually.
So why don’t we have herd immunity to other diseases?
Influenza is the most serious viral disease (population-wise, anyway) that still circulates every year in high-income countries. Although we do have vaccines for flu, we won’t be looking forward to herd immunity with the vaccines we currently use because influenza loves to mutate. It changes all the time in a process called antigenic drift. And it tends to change in a way that means last year’s vaccine doesn’t help your immune system identify this year’s version of the virus. This means we need to update our immune system as to what to be watching out for once a year.
On top of that, influenza viruses circulate in several other animal populations, especially birds and pigs. And the bird and pig versions of influenza can meet up and swap genetic material with human versions in a way that is fairly unique, leading to *very* new types of flu–types that would take our immune systems completely by surprise. This is the scenario that could someday lead to an influenza pandemic. The Great Influenza pandemic of 1918 probably originated in a wild bird population in North America (though, no one really knows for sure).
Scientists are currently developing a “universal flu vaccine” that would cover all strains, and all possible future strains. That would be amazing news. Some trials are currently underway. Until then, herd immunity for influenza is not possible because new strains will constantly be escaping last years’ vaccine formulas. It would sure help if more people GOT their flu vaccine though… we only get about half of people covered in the United States each year. And every new case is another chance for the virus to mutate. Really high vaccine coverage could slow down the drift process and make this year’s vaccine last longer.
The common cold is not caused by one virus. In fact, the set of symptoms we call “the common cold” can be caused by dozens of different viruses. Vaccines are extremely specific to a single virus, and sometimes even virus strain (as with flu). So a vaccine for the common cold would actually need to be many vaccines! Most cases of the common cold are caused by rhinoviruses, but there are more than 160 different strains of rhinovirus circulating. So far, developing a universal rhinovirus vaccine–one that would cover all strains–has eluded vaccine developers. But as with flu, scientists are trying.
You didn’t ask about HIV, but we’ll answer anyway. Scientists have been trying to develop a vaccine for HIV since the disease was discovered. But it’s a very difficult nut to crack. HIV is able to hide from the body’s immune system. A vaccine works by giving our immune system instructions as to what it should watch out for. With HIV, there are no instructions to give. It’s like giving instructions to look for something invisible.
We have reached herd immunity with many (many!) diseases! 100 years ago, infectious disease was the leading cause of death in the United States. Measles, mumps, rubella, HIB, polio, chicken pox, smallpox, whooping cough (pertussis), diphtheria, tetanus, rabies, and yellow fever used to circulate widely in the United States and all over the world.
Declines are not due to herd immunity for all of these. For example, we’ll never get to herd immunity for yellow fever, because it’s spread by mosquitoes. Unless you could somehow vaccinate the mosquitoes, you can’t protect the unvaccinated from yellow fever. Same goes for tetanus–you can’t vaccinate rusty nails. For these diseases, we’ll always need to rely on individual-level protection and behavioral prevention (like wearing bug spray and shoes). Your vaccine protects you and no one else.
When we achieve herd immunity, it means that *even the people who are unvaccinated* will never get the disease, because they’ll simply never be exposed to it. Smallpox is the ultimate example. We did such a great job of controlling smallpox that we don’t even vaccinate for it any more. And yet the worldwide number of smallpox cases every year since 1980 has been: zero. Smallpox has been *eradicated* from planet Earth through vaccination. Herd immunity slam-dunk.