What is herd immunity, and why is it so hard to reach for COVID-19?
Herd immunity means enough people are immune (through vaccination or prior infection) that the virus can't spread easily, protecting even those who aren't immune. The key number is the "herd immunity threshold" (HIT)—the percentage of the population that must be immune. For COVID-19, this threshold isn't fixed; it changes with the virus variant and how people behave. One study estimated that for the original SARS-CoV-2 virus in Spain, the HIT ranged from 28% to 68%, depending on the method used to calculate it [3]. But when the Delta variant arrived, that range jumped to 4.02–8.96 for the basic reproduction number, pushing the HIT upper bound to 90% [3]. That means nearly everyone would need to be immune to stop Delta's spread—a much taller order.
Another challenge is that COVID-19 doesn't behave like measles, where one infection or vaccine gives lifelong immunity. For SARS-CoV-2, immunity wanes over time and the virus mutates, so reinfections are common. One paper argues that for such pathogens, mass vaccination can only be expected to delay new infections, not prevent them entirely, which weakens the moral obligation to vaccinate for herd immunity and makes coercive policies less justifiable [5].
What vaccination rates are needed, and what gets in the way?
A mathematical model using U.S. data found that herd immunity could be achieved if at least 60% of the population were fully vaccinated with the Pfizer or Moderna vaccines [4]. That study also showed that combining vaccination with continued mask-wearing and other non-pharmaceutical interventions (like social distancing) significantly shortened the time to eliminate the pandemic [4]. So even with a good vaccine, behavior still matters.
But reaching even 60% is a struggle. Vaccine hesitancy is a major barrier. In a large Swedish survey of unvaccinated people, giving a high herd immunity threshold (90%) actually made some people more willing to vaccinate (because they thought many others would get the shot), but it also made others less willing (because the goal seemed unattainable) [1]. This shows that how we talk about herd immunity can backfire. On the positive side, a U.K. study found that simply informing people about herd immunity through vaccination increased their intention to get vaccinated, and so did appealing to empathy for vulnerable people [2]. Tailored outreach programs—like the one in South West England that vaccinated nearly 8,000 hard-to-reach people (homeless, refugees, ethnic minorities)—show that targeted efforts can boost uptake [7].
The mix of vaccines also matters. Most herd immunity formulas assume one vaccine type, but in reality, countries use multiple vaccines with different effectiveness. One study showed that using a combination of vaccines can change the herd immunity threshold, and that policymakers need to update their calculations when multiple variants are circulating [8]. So the simple "70% vaccinated" target is an oversimplification.
Can we still get there with new variants?
New variants make herd immunity even harder. The Delta variant, for example, was more transmissible, and data from a Massachusetts outbreak suggested that while vaccination remained highly protective against severe illness, it might not be enough on its own to stop the spread of Delta [6]. This means that even in highly vaccinated populations, the virus can still circulate among the unvaccinated and even some vaccinated people, making elimination unlikely.
A study of eight countries with high vaccination rates (over 60 doses per 100 people) found that vaccination did reduce infection rates, but the pattern varied: some countries saw an inverted U-shaped trend (infections rose then fell), while others saw an L-shaped trend (steady decline) [9]. The turning point—when infections started to drop—occurred at vaccination rates between 1.46 and 50.91 doses per 100 people, showing that the relationship isn't simple [9]. The bottom line: vaccination is powerful, but herd immunity as a permanent, stable state is probably not achievable for COVID-19. Instead, the goal is to keep cases low enough that hospitals aren't overwhelmed and vulnerable people are protected.
About These Sources
This answer is built on 9 peer-reviewed studies — published from 2021 to 2023, 6 in Q1 journals, collectively cited 394 times — selected as the most relevant from 10 studies that passed quality screening, drawn from 50 papers retrieved from a database of over 500 million.
Sources used in this answer
The effect of herd immunity thresholds on willingness to vaccinate
In a representative sample of 1,540 unvaccinated Swedes, presenting a high (90%) vs. low (60%) herd immunity threshold had opposing effects: it increased vaccination willingness in some (by making them think more others would vaccinate) but decreased it in others (by making the goal seem unattainable) [1].
Information about herd immunity through vaccination and empathy promote COVID-19 vaccination intentions.
Across two online studies with 2,315 U.K. participants, both information about herd immunity through vaccination and empathy for vulnerable people independently and causally increased COVID-19 vaccination intentions [2].
Caveats on COVID-19 herd immunity threshold: the Spain case
For Spain, the herd immunity threshold for the original SARS-CoV-2 variant ranged from 28% to 68% depending on estimation method, but the Delta variant pushed the upper bound to 90% [3].
Toward Achieving a Vaccine-Derived Herd Immunity Threshold for COVID-19 in the U.S.
A mathematical model using U.S. data showed that vaccine-derived herd immunity for COVID-19 is achievable if at least 60% of the population is fully vaccinated with Pfizer or Moderna, and combining vaccination with moderate non-pharmaceutical interventions significantly speeds elimination [4].
Herd immunity, vaccination and moral obligation
For pathogens like SARS-CoV-2 where reinfection is common due to waning immunity and antigenic variation, mass vaccination can only delay new infections, not prevent them, making the moral obligation to vaccinate for herd immunity weaker and coercive policies less justifiable [5].
Covid-19: Delta infections threaten herd immunity vaccine strategy
Data from a Massachusetts outbreak suggested that while vaccination remains highly protective against severe COVID-19 from the Delta variant, it may not be sufficient on its own to stop the variant's spread [6].
Leaving No One Behind: Interventions and Outcomes of the COVID-19 Vaccine Maximising Uptake Programme
A targeted outreach program in South West England vaccinated 7,979 high-risk individuals (homeless, refugees, ethnic minorities, people with disabilities) from February to August 2021, demonstrating that tailored communication and engagement can improve uptake among hard-to-reach groups [8].
The herd-immunity threshold must be updated for multi-vaccine strategies and multiple variants
The classic herd immunity threshold, defined for a single vaccine, is invalid when multiple vaccines or multiple variants are in play; the threshold must be updated for multi-vaccine strategies and coexisting variants [9].
The Effect of Vaccination Rates on the Infection of COVID-19 under the Vaccination Rate below the Herd Immunity Threshold
In eight countries with high vaccination rates (over 60 doses per 100 people), vaccination reduced infection rates, but the pattern varied (inverted U-shaped or L-shaped), with the turning point occurring at vaccination rates between 1.46 and 50.91 doses per 100 people [10].