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In the seemingly endless March 2020, it almost seemed like the pandemic would never end. A one-year waiting period for a vaccine was considered very optimistic and there were no guarantees. But the vaccines arrived faster than expected and we are on target for an unprecedented immunization campaign.
Countries around the world are beginning to see the end of the pandemic, or at least some are returning to a degree of normalcy. For that to happen, we need to achieve some form of herd immunity or at least get very close to it.
So how much population do we need to vaccinate to achieve herd immunity? Estimates fall between 60% and 90%. No one knows for sure, but this is what we do know and why all of this matters.
What is herd immunity anyway?
Herd immunity is not a new concept; we’ve addressed it long before the COVID-19 pandemic. Herd immunity occurs when enough people are immunized that the spread of infection throughout the community is no longer possible. When herd immunity is achieved, people within the community are protected, even those without protection.
It is essentially a way of protecting everyone in society, including those who cannot develop immunity for some reason or cannot be vaccinated for medical reasons (immunosuppressed people, for example).
Herd immunity is achieved when a sufficiently large portion of the population becomes immune and the virus can no longer find routes of transmission. That’s one way of saying that we keep the virus under control and that it will no longer be an important part of our lives.
Herd immunity can be achieved through vaccination (the easy way) or by contracting the disease (the hard way). The first is definitely better than the second. For some diseases that mutate frequently (like the flu) and can reinfect you, herd immunity can never be achieved through infection alone, and for coronavirus it is not an option either.
Herd immunity, but only by vaccination
In the early days of the pandemic, herd immunity seemed like a good idea … to some. Yet despite being popularized by politicians like Donald Trump and Brazilian Jair Bolsonaro, the idea of achieving ‘herd immunity’ by exposing people to the virus is unethical and, at best, scientifically. questionable. It would be something between very risky and catastrophic.
We are already seeing it happen: in the Brazilian city of Manaus, 76% of the population had been infected with SARS-CoV-2 in October 2020. However, in January 2021, a further increase in hospitalizations was reported in Manaus, with a dangerous viral mutation emerging in the process. Whether due to lowering antibodies, a new mutation, or some other explanation, it is a case study that clearly shows that allowing the virus to spread freely is not a solution.
Most researchers also agree that letting the virus “run its course” through the population is a recipe for disaster. Not only will it end up killing and injuring a lot of people, but at the end of the day, it could still end up being wasted because we don’t know how long immunity lasts.
There are many unknowns when it comes to natural immunity, but let’s say, for the sake of argument, that immunity would last an average of 12 months. If immunity waned after one year, we would have to start over after one year and we would not have achieved as much. However, with a vaccine, that would be a very different situation, since we could repeat vaccines annually.
It is not just theory, this is more or less what we are doing with flu viruses. We are a long, long way from achieving herd immunity against the flu, but many people get immunized with a vaccine every year and it is working.
Presumably, we will have to do the same with COVID-19 for a while. While some countries may achieve herd immunity as of 2021, it is unlikely that the entire planet will be able to achieve it, which means that the virus will still go crazy in some parts of the world and have the opportunity to mutate and change (which means we would have to change vaccines and receive new injections every year).
How many people need to get vaccinated against COVID-19 to get things back to normal?
So far, we have spoken in general terms, but let’s get closer to COVID-19. How many people do we need to vaccinate to achieve herd immunity against COVID-19?
The percentage of people who should be immune to achieve herd immunity varies with each disease, especially depending on how contagious it is. For measles, the herd immunity threshold requires that around 95% of the population be vaccinated. For polio, it is 80%. For the flu, some estimates put it at 44%. For COVID-19… we are not really sure.
| Disease | Transmission | Herd immunity threshold |
|---|---|---|
| Measles | Airborne | 92–95% |
| Whooping cough | Droplet in the air | 92–94% |
| Diphtheria | Saliva | 83–86% |
| Rubella | Droplet in the air | 83–86% |
| Smallpox | Droplet in the air | 80–86% |
| Polio | Fecal-oral route | 80–86% |
| Mumps | Droplet in the air | 75–86% |
| SARS | Droplet in the air | 50–80% |
| Ebola | Body fluids | 33–60% |
| Influenza | Droplet in the air | 33–44% |
Initial estimates establish a tentative threshold for herd immunity at 67%, with a reproduction number of 3 (each person who contracts the virus would pass it on to another 3 in a population where all individuals are susceptible to infection). We’re still not sure what the breeding number is, but during the early part of the pandemic, 60-70% was largely distributed by researchers.
However, while the World Health Organization still cites that range and is still cited, US officials have quietly started to move the figure up. Anthony Fauci started saying “70.75 percent” in television interviews and, in an interview with CNBC News, he said “75, 80, 85 percent” and “75 to 80 or more percent.” Quoted by the NY Times, Fauci would “bet his house” that COVID-19 is not as contagious as measles, and believes that the figure needed for herd immunity is not as high as 90%. But it’s probably a little over 70% too. We must also take into account the fact that some vaccines seem to work somewhat worse than others, and in this case, the effect should be offset by a higher vaccination rate.
Simply put, the herd immunity threshold for COVID-19 appears to be around 70-80%, but we are not entirely sure where it is.
Mutations: another incentive for rapid vaccination
We do not necessarily you need herd immunity to get things back to normal. The more people we vaccinate, the more people become immune, and the more the risk is reduced. Herd immunity ensures that we protect a vulnerable part of the population, but it has another advantage: it reduces the risk of mutation.
Viruses change and mutate all the time, some faster, some slower. For the most part, these mutations don’t represent much. But every now and then a dangerous mutation emerges, a mutation that can make the virus more contagious, more dangerous, or more resistant to vaccines. The idea of vaccinating a large portion of the population only for the virus to mutate and bypass the vaccine is one of the worst possible scenarios. That is why vaccination is not really a national race: Viruses really have no borders. If some nations achieve herd immunity and others do not, mutations can still emerge and threaten the vaccinated nation.
Viruses cannot mutate if they cannot replicate. The more people we vaccinate, the closer we get not only to herd immunity but also to ending the pandemic.
Are we there yet?
We are not even close to herd immunity at the moment, but it is quite plausible that by the end of this year, several countries will be able to achieve it. If we want to vaccinate 80% of the population in one year, we need to vaccinate 6.6% of the population every month. In the United States, for example, that means vaccinating 21 million a month.
Despite a difficult start, it can be done. Assuming vaccine vacillation doesn’t start to take its toll (recent polls show it’s plummeting), our main obstacles are production and distribution chains. We already have several safe, approved and working vaccines. Production is increasing, logistics is already being implemented, we could get there.
However, disparities (both national and international) are likely to plague vaccination campaigns. The dangers of vaccine nationalism are very present and striving to be the first to inoculate your population will accomplish little if others are not vaccinated as a result. The pandemic does not affect everyone equally, but it does affect everyone, and only together can we truly overcome it.
