Story at a glance
- Herd immunity is a situation in which enough people in a population have immunity to a disease to prevent it from spreading if introduced.
- For the coronavirus, experts have estimated about 60 to 70 percent of people need to be immune to achieve herd immunity.
- A new model estimates the percentage could be lower, at around 20 percent, depending on human behavior.
We’re now more than half a year with the SARS-CoV-2 coronavirus and COVID-19 pandemic, and researchers are talking about whether herd immunity can be achieved naturally without a vaccine. That would require millions of people getting infected by the virus and many deaths. What goes into calculating how many people need immunity to get to herd immunity? And is it valuable to think in these terms?
Earlier during the coronavirus pandemic, researchers estimated that 70 percent of a population would need to have immunity to achieve herd immunity; more recent estimates give a range. For example, in February one group estimated the threshold to be between 40 to 70 percent. More recently, a group of researchers ran different models that suggest the threshold is less than 20 percent, according to The Atlantic.
What do these percentages mean? First of all, when health experts talk about herd immunity, it is nearly always in situations where achieving immunity is predictable, like with vaccines, or with diseases that have become common in humans. Many vaccines confer lifetime immunity, meaning you don’t need to get the vaccine again. Although for some diseases, boosters are encouraged. For example, tetanus shots are recommended every 10 years.
To get the percentages, researchers calculate the minimum proportion of the population that needs immunity to prevent spread within the community. This depends on how well that particular disease spreads, what age groups spread it most, the method of transmission as well as other factors. Enough people need to have immunity to lower the rate of transmission so the disease cannot increase within the population.
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When a vaccine exists, health experts aim to vaccinate the minimum percentage of the population to achieve herd immunity. The experts know how effective the vaccine is and that makes it predictable and useful. However, when you are talking about a new disease that is spreading naturally within a population, there may not be enough information to reliably make those calculations and give a useful herd immunity level.
For the coronavirus, it’s unlikely that the real herd immunity threshold will be as low as 20 percent, but it’s possible depending on the variability of susceptibility to the virus, says Tom Britton, who is part of the research group that made the 40 to 70 percent estimate, to The Atlantic. In a recent study, Britton and collaborators state that disease-induced herd immunity level could be as low as 43 percent.
Some health officials have mentioned achieving herd immunity naturally through infection. For example, some in Sweden have talked about reaching herd immunity without drastic control or lockdown measures. “The concept of creating herd immunity by infection is similar to creating it by vaccination,” says Bharat Pankhania at the University of Exeter Medical School. “The difference is that when you vaccinate, you are using tried, tested and extremely safe vaccinations.”
A word about models
The models mentioned so far with lower thresholds include variation in human behavior, sometimes referred to as heterogeneity. It can also be described as how people mix or connect within a population. This is different from models that simplify human behavior, treating it as homogenous. Models with homogeneous behavior put the herd immunity level at around 60 percent.
When behavior is treated as heterogenous in models, the threshold tends to be lower because behavior affects how the disease may spread in the population, according to Gabriela Gomes of University of Strathclyde whose group modeled the 20 percent threshold and who spoke to The Atlantic. For example, behavior could be different across age cohorts. Another way to differentiate is by level of social activity.
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That’s not to say one approach is better than another. Think of models as tools. A simpler model may have a single purpose, but that may make it lightweight and easy to use. A more complex model might take more data and inputs, making it clunky and more difficult to understand or interpret.
There’s not one model to rule them all. Many researchers will tell you that models are never right but they can be helpful. In the case of COVID-19, we’re seeing many models and results coming out at a rapid pace. It’s important to remember that as the pandemic is ongoing, so is the research and scientific discovery around it. It’s wise to refrain from drawing conclusions too early.
Do we get immunity?
Another factor to consider is whether being infected by the coronavirus and recovering will then result in immunity. For many diseases, this is true so we may take for granted that it’s the normal course of things for infections. Our immune systems are pretty complex and there’s a lot of variation in how it responds to pathogens. As of this writing, health experts and researchers are collecting as much data as they can to understand this but cannot definitively say whether someone who has recovered has immunity.
When we can’t tell if immunity is achieved after recovery and for how long, like with SARS-CoV-2, we really don’t know much about where we are on the road toward herd immunity. We are seeing new antibody studies, such as in Spain where 5 percent of people tested were found to have antibodies to the coronavirus. Another study in New York reports that in a group monitored from Feb. 23 through April 19 the seroprevalence, or presence of antibodies, reached 19.3 percent. These studies are important and could be useful in the future when we understand more about COVID-19, but for now they aren’t helpful in telling us if we’re closer to herd immunity.
Considering what we don’t know yet, a positive test for antibodies does not indicate that the person has immunity to SARS-CoV-2. It might help us to understand how many people were potentially infected during a certain time period, but it can’t tell us if that person will be protected in the future. “Once we know how many people actually got the infection, we can adjust our numbers in terms of the mortality or what we call case fatality,” says Nahid Bhadelia of Boston University School of Medicine. There’s also the possibility of false positives and false negatives that could skew the results. More long-term research needs to be done to understand if someone who has antibodies has immunity and if so how long that immunity lasts.
Stories of reinfections
A recent study posted as a preprint, meaning it has yet to be peer-reviewed for publication in an academic journal, suggests that a large proportion of people who recover from SARS-CoV-2 have antibodies for only a short period of time. The researchers collected samples up to 94 days after onset of symptoms.
Experts are hopeful that people who have antibodies may have some protection against future reinfection, but we don’t know that for sure. Current antibody tests are only looking for certain antibodies, so there are other things to search for in the immune system, like T cells, that are specific to SARS-CoV-2.
“We know from SARS infection (SARS-CoV-1) that T cells persisted many years longer than antibodies so we need to see whether that may also be the case after COVID-19,” says Mala Maini, a professor of viral immunology and consultant physician at University College London. “Follow-up studies will tell us much more about the real-life level of protective immunity people who’ve had COVID-19 show when they re-encounter the virus.”
The study reinforces the message that we can’t assume someone who has had COVID-19 and tests positive for antibodies can’t get it again.
However, there have been stories about people getting reinfected with SARS-CoV-2 and getting sick again. Earlier in the pandemic, health officials thought reinfection occurred in South Korea, but that turned out to be detection of fragments of coronavirus from samples, not true reinfection. Recent anecdotes suggest that people who tested positive months ago and cleared the virus are getting it again and suffering a more severe case of COVID-19 the second time around. These cases are different from “long haulers,” who are experiencing persistent symptoms over several months.
It’s too early to tell if this is happening more widely or if these are outlier cases.
Unfortunately, no hard and fast numbers
In the end, the percentages for herd immunity are somewhat arbitrary. It depends on what the real world conditions are, and especially how people are behaving. Models can only account for so much, otherwise they’ll become too complex and won’t be useful. In general, herd immunity percentages are minimum goals to aim for and best achieved through vaccination. And this is all assuming that we can achieve immunity at all.
Herd immunity might be attainable for the coronavirus without a vaccine if many people get infected, and we might have a successful vaccine within the next year. We don’t know for sure about which might happen first, and getting caught up in that debate may not be helpful. In the meantime, we should do what we can, which is be smart about containing the spread of the virus and working to keep the number of cases low.
For up-to-date information about COVID-19, check the websites of the Centers for Disease Control and Prevention and the World Health Organization. For updated global case counts, check this page maintained by Johns Hopkins University.
You can follow Chia-Yi Hou on Twitter.
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