Getting back to ‘normal’ in Oregon means getting to COVID-19 herd immunity

By Jes Burns (OPB)
Feb. 24, 2021 2 p.m.

Explaining herd immunity and how we get there.

It was a bumpy start, but vaccines for COVID-19 are now being administered with consistency across Oregon. As of Feb. 22, more than 270,000 Oregonians were fully vaccinated – that’s about 7% of the population.


But to get back to “normal” — with businesses and schools open without restrictions, no masks, no social distancing — that number will have to go up. Way up.

“I know some people who are tired — not just some people — a lot of people who are really tired of the pandemic and all the restrictions. They’re hoping there’s a magic bullet approach — that the vaccine will be a magic bullet,” said Chunhuei Chi, a public health professor at Oregon State University.

But vaccines aren’t going to solve all of our COVID-19 problems this week, this month, or maybe even this year.

Getting past the pandemic – and back to normal — means getting to herd immunity. It’s a goal — talked about as a percent — that’s been shared around with giddy anticipation. But how we get there is more complex than a simple number conveys.

What is herd immunity?

Herd immunity happens when enough people in a population become immune to an infectious disease that it is unlikely to continue spreading.

Viruses have to infect people in order to spread. If someone who is infected with a virus comes into contact with five people and four of them are immune, then the virus can only spread to one other person. With enough immune people in the mix, the virus won’t have anyone available to infect. If there come to be enough immune people, the disease will eventually vanish from a population — like smallpox. However, coronavirus is expected to become endemic, that is to say, it’s expected to stick around at low levels for quite a while.

Once herd immunity is reached, the people most vulnerable to the disease (those who aren’t immune) likely won’t ever come into contact with someone with the virus. They will be protected by the larger immunity of the herd.

Herd immunity is usually given as a percent – or more reasonably, a percentage range – of the population that needs to be immune to provide community protection.

How long have we known about it?

The term herd immunity has been around for more than a century. Early on it was used by veterinary scientists. One of the earliest known mentions can be found in a 1921 report out of Oregon Agricultural College (now Oregon State University) about Bang’s disease in cows. Bang’s is a nasty contagious bacterial infection that causes cows to abort their calves. The bacteria can also be spread to humans.

“It’s estimated that the infectious abortion and sterility in cattle cost the State more than one million dollars annually ($14.5 million in today’s currency),” the report stated, adding that the Department of Veterinary Medicine had focused much of its energy on studying “the possibility of the disease disappearing through building up of herd immunity.”

The term started to be used more widely to talk about infectious disease in humans in the decades that followed.

How do we get immunity?

People become immune to a disease when their bodies generate antibodies capable of fending off an invading virus, bacteria, fungus or toxin. In the case of COVID-19, your body learns to fight it by first being exposed to the coronavirus. There are two primary ways this happens:

It’s still unknown how long natural and vaccine-derived immunity will last (more below).

Do we need the vaccine to get there?

Technically, no. For most of human history, humans got to herd immunity without the use of vaccines. And for most of human history, people died in large numbers or were debilitated by infectious disease — everything from plague to polio to smallpox.

So we can get there through natural immunity — if we’re willing to allow people to die preventable deaths.

“I call it ‘herd immunity the hard way,’” said Oregon Health and Science University professor Mark Slifka, whose research focuses on vaccines.

By one count, more than a million people in the United States would have to die of COVID-19 before half the population gains natural immunity.

With vaccines, this doesn’t have to happen.

“Doctors and medical scientists tell us that a vaccine is the safest and most effective way to stop the spread of the virus,” said Oregon Health Authority Director Patrick Allen as vaccination rates in the state were starting to ramp up.

Is the herd immunity goal the same for all diseases?

No. The percentage of people that need immunity to reach herd immunity hinges on how infectious a disease is. The way epidemiologists calculate this is by determining the basic reproduction number, often called the R0 (R-naught) or R-value. This number answers the question: How many others is each sick person expected to infect if nothing is done to slow or stop the spread? Once again, this is often given as a range of numbers because a disease will spread differently depending on factors like weather and population density.

A disease like Ebola has an R-value of around 2. One infected person on average would infect two others. For something highly contagious like whooping cough, one person would pass it on to 15 to 17 other people. At an R-value of 1, case numbers plateau. Below that and case numbers fall.


There are other versions of the R-value use as well. When you see Re or Rt, that refers to the real-time rate of spread of a disease – one that reflects things like the use of masks, social distancing and quarantine. For a couple of months now, Oregon has been hovering at or just below the Re=1 mark, which means in real time, COVID-19 rates flattened and have started to drop.

But in calculating herd immunity, R0 is used. To get back to normal, we need to think about disease spread under normal conditions.

“We don’t want to keep wearing masks. We want to actually go to football games again someday. And so that shows you the level of immunity you need so you no longer have to have all these mitigation factors,” Slifka said. “We don’t have to wear masks to stop measles. And the reason why is because we have herd immunity for measles.”

How do you calculate herd immunity?

The basic math is relatively straightforward. It starts with a formula: 1-(1/R)

For COVID-19, a good middle-of-the-road R-value is 3.3.

Using the formula, the herd immunity threshold for coronavirus would be approximately 70%.

“If we get 70 to 85% of the country vaccinated by the end of the summer, middle of the summer, I believe by the time we get to the fall, we will be approaching a degree of normality, not perfectly normal, but one that will take a lot of pressure off the American public,” said Dr. Anthony Fauci in his first press conference as President Joe Biden’s top medical adviser on COVID-19.

How does this work out for Oregon?

With a population topping 4 million, the state would need to have just under 3 million people immune to COVID-19 to reach herd immunity. In mid-February, there were about 150,000 confirmed coronavirus infections. That sets our vaccination goal at around 2.8 million people to hit that 70%.

At the state’s early-February average vaccine rate of 16,000 per day, we’ll get there right around Christmas of this year. This date would move forward if that daily rate increases.

But hitting this goal is going to be more complicated than it seems. The vaccine hasn’t been approved for kids and teens under the age of 16. That’s about 17% of the state’s population who can’t get vaccinated. There will also be a small portion of Oregonians who can’t get vaccinated because of underlying health conditions.

This means that about 90% of healthy adult Oregonians will have to get vaccinated to hit that herd immunity threshold.

Why do we hear so many different estimates for herd immunity?

Over the past few months, Fauci and other public health officials have given several different vaccination targets for herd immunity, usually between 65% and 85%. Officials tend to aim high when it comes to public health goals because of the uncertainty involved in getting people to act.

“Welcome to public health,” Slifka said. “If you want to get 75% people vaccinated, you need to ask 80 or 90% of people to get vaccinated. You probably won’t hit 90%, but then you’ll hit that 75% that you really need.”

In addition, there’s quite a bit of uncertainty. We don’t know exactly how infectious COVID-19 is (we have an estimated range), and there are adjustments that need to be considered when it comes to immunity, vaccine effectiveness, and virus behavior. These will have the effect of increasing the amount of time it will take to achieve herd immunity:

  • Vaccine effectiveness: The COVID-19 vaccines top out at 95% effective. This means you have to add at least 5% to your vaccination goals to meet herd immunity targets — and more depending on the mix of vaccines with a lower effectiveness rate.
  • Vaccine duration: One thing that was lost in the rush to develop and test the new vaccines for COVID-19 – taking months instead of years — was the time needed to determine how long the vaccines would provide protection. Both Moderna and Pfizer-BioNTech are developing booster shots already. It’s possible that some people will start to lose vaccine immunity before the herd is protected. Slifka says annual booster shots for these vaccines will probably be needed.
  • Natural immunity duration: It’s unknown how long the natural immunity you get from catching COVID-19 will last. Because humans usually keep immunity to other coronavirus-caused sicknesses (the common cold) for about a year, Slifka suspects our immunity to this virus will be similar.
  • Virus variants: Mutations of coronavirus are already emerging, and will continue to emerge as time passes. Vaccines may not be as effective against some variants. Others are more contagious. For example, the U.K. variant has been found to add .7 the R-value of the disease. This difference in infectiousness would raise the herd immunity target from 70% to 75%.

Is there any way to speed up the process?

Yes! One way is to vaccinate people faster. With vaccine manufacturing continuing to ramp up and new vaccines likely to come online over the next few months, this will likely start to happen. The federal government recently upped Oregon’s supply of vaccines, so it’s already starting.

Another factor that officials can’t fully rely on, but is nonetheless playing out in the background, is that the number of people who are naturally immune to COVID-19 is much higher than the official counts indicate. Testing in the United States has been lacking and largely focused on symptomatic people and those with known exposures to coronavirus. Those infected who don’t have symptoms are largely undercounted.

According to the Centers for Disease Control and Prevention, there are actually 4.6 times the official number of COVID-19 infections in the U.S. Because these cases are unknown, these people will still be in the vaccine pool. But it does provide a bit of cushion, bringing us closer to the goal.

What does life look like between now and ‘normal?’

In Oregon, mask mandates, social distancing measures and other tactics designed to slow the spread of COVID-19 have largely worked. The real-time infectious rate (R-value) for the virus is below 1 and the state’s daily case numbers are slowly dropping.

Add vaccines, then things start looking even better.

“These things can work together,” Slifka said. “The whole idea is if we do the social distancing where we have a very slow (spread). Then you throw in the vaccinations on top of it. We can drop down to a .9, .8… maybe down to a .2, and then it starts burning itself out … it’s going to drop like a rock.”

At that point, the state can start relaxing restrictions.

But caution will be necessary. Reopening will require managing the balance of a giant see-saw. If officials ease up on restrictions too much or too soon, the vaccines won’t have had time to make up the difference. Cases may start creeping back up.

Looking ahead, top public health officials are predicting the weight of the pandemic will start to lift by mid-to-late summer.

“It will be incremental. It’s going to take months before we slowly resume normal activity,” OSU’s Chi said.


Tags: Science & Environment, Health