How close are we to herd immunity for COVID-19?
The vaccines are effective 鈥� but we still have a long way to go in the U.S.
The vaccines are effective 鈥� but we still have a long way to go in the U.S.
The vaccines are effective 鈥� but we still have a long way to go in the U.S.
There鈥檚 one phrase that regularly comes up in the discussion about life in a post-COVID world: herd immunity. After all, this form of mass protection has been essential in slowing and squashing the spread of various contagious diseases in the past, including polio, measles, chicken pox and mumps, says , a primary care physician at Mercy Medical Center in Baltimore.
A community gains herd immunity when a significant portion of the population (the 鈥渉erd鈥�) becomes immune to an infectious disease, according to the (CDC). A person can achieve that immunity through vaccination or building an immune response due to prior infection, making person-to-person spread of the disease unlikely.
When it comes to COVID-19, the road to herd immunity isn鈥檛 simple. Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said in late February on the podcast that the U.S. will 鈥渉opefully鈥� reach herd immunity 鈥渂y the end of the summer and the early fall鈥� in 2021.
A recent CNN analysis estimates that 70% of the U.S. population could be fully vaccinated against COVID-19 by the end of July, and that 85% may be vaccinated by mid-September鈥攂ut we鈥檙e nowhere near that number just yet.
Even with highly infectious variants spreading, experts agree that timeline may be possible if vaccination rates continue to ramp up. 鈥淚 do think in late summer we will be closer to herd immunity,鈥� says infectious disease expert , senior scholar at the Johns Hopkins Center for Health Security. Here, experts break down how far we have left to go.
How does herd immunity work, exactly?
First, it鈥檚 important to understand how we build immunity to various contagious illnesses in the first place. When you鈥檙e exposed to a disease-causing pathogen, like the , your body launches an to attack it.
Whether you develop symptoms or not, your body will produce unique antibodies 鈥� proteins that fight against what your system sees as invaders 鈥� that will help you ward off the harmful pathogen if you encounter it again. This protection can be established through natural infection or through an .
So, when a large amount of people become immune to an infectious agent, it makes it that much harder for people who 补谤别苍鈥檛 immune to contract. The virus can鈥檛 break through the herd, and even people who can鈥檛 be vaccinated, like newborns, people with certain health conditions or people with severe allergies to a vaccine ingredient, are given some protection because the disease doesn鈥檛 have a lot of opportunity to spread, the CDC says.
A good example: from the U.S. in 2000 due to herd immunity. While clusters of cases show up here and there, the risk for the majority of the population is still low thanks to high rates of vaccination, the CDC says.
Herd immunity varies by disease, though. 鈥淭he percentage of people that need to be immunized depends on the particular disease. It could be anywhere from 30% to 90%,鈥� Dr. Besser says.
Is herd immunity possible with COVID-19?
That鈥檚 the hope 鈥� but it has to be acquired safely and natural infection isn鈥檛 the answer due to the high risk, experts say. A vaccine is the 鈥渟urest way鈥� to develop herd immunity in a population, says Dr. Adalja. While some immunity will come from natural infection鈥攖hose who fell ill with COVID-19 and recovered from it 鈥� experts don鈥檛 think 鈥渘atural infection is sufficient to slow the virus,鈥� he says, as it has killed more than in the U.S. alone at the time of publication.
Plus, COVID-19 is caused by a newly discovered coronavirus (yes, a year old is still new), and experts 补谤别苍鈥檛 sure once a person naturally builds antibodies. 鈥淚f you get a measles infection, the protection lasts for your entire life, but that鈥檚 not the case with some other infections. For COVID-19, it鈥檚 currently an open question,鈥� says , an infectious disease specialist and professor at the Vanderbilt University School of Medicine. Data so far does seem to suggest that antibodies from a natural infection don鈥檛 last forever, he adds.
Measles is the most infectious virus that experts know of, Dr. Schaffner says, and 鈥渕ore than 90% of the population must be immunized against the virus,鈥� for herd immunity to have some impact. Most respiratory viruses 补谤别苍鈥檛 as infectious as measles, though, and those generally need about 85% of a protected population to achieve herd immunity.
For COVID-19, estimates given by Dr. Fauci range from 70% to 85%, but that number could grow higher as variants continue to spread. The emphasizes that we do not yet know how many people have to be vaccinated against COVID-19 before herd immunity is reached.
The available COVID-19 vaccines are helping us get closer to herd immunity.
鈥淰accines induce immunity, and the more people who are vaccinated, the closer you come to that threshold of herd immunity,鈥� Dr. Adalja says. To date, more than 100 million vaccine doses have been administered in the U.S., according to , reaching a little more than 22% of the population.
Nearly 30 million people in the U.S. 鈥� or 9% of the population 鈥� has had COVID-19, according to data collected by . But Dr. Adalja says it鈥檚 important to , too, to help them build up lasting antibodies and T-cell immunity (T-cells kick into action after you鈥檝e been infected with a pathogen). 鈥淰accine-induced immunity is also more robust,鈥� he says.
Reaching herd immunity with a vaccine is a little more complicated than it seems, though. Not everyone has the same risk of getting and passing on the virus 鈥� some are more likely to be infected and spread it (think: essential workers), while others may not have as high of a risk of passing it on (think: people who have been working from home and mainly staying indoors), Dr. Adalja says. 鈥淵ou may get the benefits of herd immunity before you actually cross the threshold of herd immunity if the people who are responsible for the majority of cases get vaccinated,鈥� he says.
What happens after we finally reach herd immunity for COVID-19?
Doctors think COVID-19 will eventually be a smaller threat. 鈥淚 doubt it will go away,鈥� says Dr. Richard Watkins, an infectious disease specialist and a professor of internal medicine at the Northeast Ohio Medical University. He predicts that 鈥淐OVID-19 will probably become seasonal, like the flu, with increases in the winter.鈥�
Dr. Adalja also thinks COVID-19 will become 鈥渁 much more manageable respiratory illness,鈥� and anticipates that cases will drop. 鈥淭hey鈥檒l be much lower, because the virus will have a harder time finding people to infect,鈥� he says.
Finally, keep this in mind, per Dr. Adalja: 鈥淲e should go back to some semblance of normalcy even before we reach herd immunity.鈥�