What Is Herd Immunity for COVID-19 & How Will We Know We’ve Achieved It?

Herd immunity has become a popular and important concept to help us understand our progress in combating the Covid-19 pandemic. If herd immunity is one of our goals, it’s important to understand what it is, when we will reach it, and what that will do for us.

The concept of herd-immunity makes common sense: once enough people have acquired immunity – whether acquired from vaccination or a prior infection – they’re no longer susceptible to catching it again, or at least it is very unlikely due to partial immunity. Unvaccinated persons in “the herd” will be less likely to be grazing, sneezing, and eating next to a herd-mate who can pass the disease along.

Can someone who is living in a place that has achieved “herd immunity” still get sick? Yes, because herd-immunity is a population level construct. It doesn’t apply to an individual. Someone may be less likely to contact an infectious person if they’re living in a place that has achieved herd immunity, but if they aren’t immunized, they can catch the virus (and spread it to other unvaccinated persons).

Ok, So What Does Herd Immunity Mean in a Practical Sense?

No doubt you’ve heard many statements like “experts believe we will reach herd immunity at around 70%”. What does this mean? It depends. Herd-immunity doesn’t have an agreed upon definition. It doesn’t mean that there are no cases of COVID-19 in the community. If that were the case, everyone would have to be immune, so we would require 100% of everyone to be immune and stay that way.

We think herd immunity means that disease transmission starts going down on its own (without interventions in place) because there’s enough immunity in the herd to block just enough secondary (and tertiary, etc.) transmission that it burns out the continued spread.

We can understand this by talking about R-values. We’ve all learned that if the effective R value is greater than 1, the disease is spreading exponentially. If it is less than 1 it’s declining, (also exponentially).

Ideally, policy makers would keep non-pharmaceutical interventions like mask wearing and limited indoor capacity in crowded bars, nightclubs, and restaurants until the R values are less than 1. They would then slowly relax those measures and continue to monitor the R value to make sure it stays below 1 before relaxing more measures. This would help us ease into a soft-landing that mimics the benefits of herd-immunity before we fully achieve it.

Eventually, you reach a spot where you no longer have any non-pharmaceutical interventions in place and the R value is still below 1 (cases per capita continue to fall).

Voila, you have achieved herd immunity!

How Will We Know When Arizona Achieves Herd Immunity?

Our governor and health director have already eliminated every single required mitigation measure except for vaccinating people. So, in a sense, we are at a baseline place where there are already no required mitigation measures. Yet, some people are continuing to respect distancing, wear masks in public, and are altering their routines to avoid crowded places.

Some businesses are still voluntarily following CDC recommended mitigation measures, but we’re getting close to the place where there are very few interventions in place.

For example, the DBacks home games will be played at 50% capacity beginning this week (they are not really limiting attendance because they almost never sell more than 50% of their seats).

To reach herd-immunity, we need to have case counts that are still decreasing (with an R less than 1) even after we have full stadiums, crowded bars, full classrooms, and we’ve all taken our masks off.

Right now, Arizona’s Rt (reproduction rate) is about 1.2 indicating that cases are steadily rising. The only intervention in place is vaccination, so we will likely continue to see an increase in cases until we hit the vaccination tipping point and we begin to see a decline – and herd immunity.

Will people still get sick after we reach herd immunity? Yes, absolutely! Herd-immunity only means that cases stop growing. It doesn’t mean COVID-19 has been extinguished. Who will be most likely to get sick? Well, obviously the unvaccinated folks will.

Herd immunity isn’t permanent. Antibodies and cell mediated immunity can wane over time – regardless of whether they are acquired naturally or from vaccination. When this happens, the amount of immunity in a population can decline. Novel variants can also make herd-immunity harder to reach if antibodies aren’t cross-protective.

Diseases also have an opportunity to become endemic meaning that they are always around and never fully die out. Enough disease sticks around in susceptible folks (or herd-mates) until a few lose their immunity and get sick. So, herd immunity needs to be thought about year-after-year. The only alternative is to eliminate a disease completely, which is unlikely for this coronavirus. We already have four endemic coronaviruses, and SARS-CoV2 is trying to become the fifth.

The original ASU forecasting models suggested that we will achieve herd immunity when about 75% of the people in Arizona have either been vaccinated or had been infected with the virus and recovered.

The ASU Biodesiign Institute recently modified their model because the new (and more transmissible) British B.1.1.7 variant is now the dominant strain in Arizona. Their new estimate is that we will need to get to 80-85% of Arizonans immunized or infected before we get to “herd immunity” under our definition.

Our promise to you is that we’ll continue to look at the data objectively and let you know when we think we’ve hit a threshold of a sustained downward trajectory in community spread in an environment where there is hardly any mitigation underway (except for vaccinations).

But remember, new variants could change the calculus, and we could again be placed into an environment where spread is rising again. The likelihood of that happening depends largely on how quickly the rest of the world achieves herd immunity. The longer that takes, the more likely it is that a new variant will throw a monkey wrench at us.

Tim Lant, Ph.D.

Will Humble, MPH

Director Christ & Governor Ducey Blind-side Superintendent Hoffman & the K-12 School System by Abruptly Eliminating School-based Mask Mandates

Governor Ducey and Director Christ just summarily eliminated a previous executive order that had required K-12 schools to have mask policies in place for students, teachers, staff and visitors. As of today, schools MAY have a mask mandate policy but it is no longer required.

That means that schools will need to individually keep and defend any mask requirement that they currently have in place. Many schools will drop their requirements. Those that keep their policies will have worse adherence.

In the opening statement of their media release that announces this foolish change, Director Christ and Governor Ducey state that:

In alignment with Centers for Disease Control and Prevention (CDC) guidance, Governor Doug Ducey and the Arizona Department of Health Services today rescinded orders that direct K-12 schools to require masks.

That statement is a lie and they know it. CDC’s guidance and recommendations for schools makes it clear that they urge schools to use “universal and correct usage of masks” in the K-12 school setting”.

These two are really a piece of work. They couldn’t even wait for the remainder of the school year to finish with the existing mask policies in place. Instead, they made mask wearing at schools optional with only 23 more days of school left in most districts.

They have really made some tremendously bad policy decisions over the last year. Now we can add another one to the list.

Editorial Note: Superintendent of Public Instruction Kathy Hoffman found out about this move only moments before Director Christ and Governor Ducey issued their press release announcing the Edict. Director Christ would have known about this for days, and yet failed to have the common courtesy to give Superintendent Hoffman a heads up.

Actually, common courtesy would have consisted of actually CONSULTING with Superintendent Hoffman before they issued this edict.

Shameful.

Federal Pharmacy Program Expansion Complete: Access Points All Across Arizona Now

Two weeks ago the White House announced that the Federal Retail Pharmacy Program for COVID-19 Vaccination is expanding from 17,000 participating pharmacies to nearly 40,000 stores by tomorrow. This achievement will make it so a vaccine site will be within five miles of 90% of all Americans. The participating pharmacies in the Federal Retail Pharmacy Program are in communities across the country – including 45% in the highest-need neighborhoods.

This is a very important program because these pharmacies get shipments directly from the federal government and don’t rely on meager allocations from the ADHS, who has been prioritizing allocations toward the state financed and university run mega-sites.

I just checked on www.vaccinefinder.org and found numerous convenient appointment that are available at several pharmacies near my house. 

Director Christ & Governor Ducey Blind-side Superintendent Hoffman & the K-12 School System by Abruptly Eliminating School-based Mask Mandates

Governor Ducey and Director Christ just summarily eliminated a previous executive order that had required K-12 schools to have mask policies in place for students, teachers, staff and visitors. As of today, schools MAY have a mask mandate policy but it is no longer required.

That means that schools will need to individually keep and defend any mask requirement that they currently have in place. Many schools will drop their requirements. Those that keep their policies will have worse adherence.

In the opening statement of their media release that announces this foolish change, Director Christ and Governor Ducey state that:

“In alignment with Centers for Disease Control and Prevention (CDC) guidance, Governor Doug Ducey and the Arizona Department of Health Services today rescinded orders that direct K-12 schools to require masks.”Governor Ducey

That statement is a lie and they know it. CDC’s guidance and recommendations for schools makes it clear that they urge schools to use “universal and correct usage of masks” in the K-12 school setting”.

These two are really a piece of work. They couldn’t even wait for the remainder of the school year to finish with the existing mask policies in place. Instead, they made mask wearing at schools optional with only 23 more days of school left in most districts.

They have really made some tremendously bad policy decisions over the last year. Now we can add another one to the list.

Editorial Note: Superintendent of Public Instruction Kathy Hoffman found out about this move only moments before Director Christ and Governor Ducey issued their press release announcing the Edict. Director Christ would have known about this for days, and yet failed to have the common courtesy to give Superintendent Hoffman a heads up.

Actually, common courtesy would have consisted of actually CONSULTING with Superintendent Hoffman before they issued this edict.

Shameful.

Federal Pharmacy Program Expansion Complete: Access Points All Across Arizona Now

Two weeks ago the White House announced that the Federal Retail Pharmacy Program for COVID-19 Vaccination is expanding from 17,000 participating pharmacies to nearly 40,000 stores by tomorrow. This achievement will make it so a vaccine site will be within five miles of 90% of all Americans. The participating pharmacies in the Federal Retail Pharmacy Program are in communities across the country – including 45% in the highest-need neighborhoods.

This is a very important program because these pharmacies get shipments directly from the federal government and don’t rely on meager allocations from the ADHS, who has been prioritizing allocations toward the state financed and university run mega-sites.

I just checked on www.vaccinefinder.org and found numerous convenient appointment that are available at several pharmacies near my house. 

Pima County Analysis of Contact Tracing Provides Interesting and Useful Results

Pima County has conducted an in-depth analysis of the case investigation reports during the pandemic. The detailed report ties the case investigation reports to the various policies that were implemented by both Pima County and the state during the pandemic.

The analysis provides evidence that many of the interventions were successful. Data from the last 3 weeks provides an early indication that a larger share of recent cases originated in closed indoor environments that formerly had some interventions in place- which were abruptly removed by the Governor and Director Christ.

I encourage you to take a look a their analysis this week. Here is the report

FDA & CDC Pause the Administration of the Johnson & Johnson (Janssen) Vaccine Pending Data Review

More than 6.8M doses of the Johnson & Johnson vaccine have been administered in the U.S. The enhanced surveillance system that’s in place to detect adverse events found 6 cases of a type of blood clot called cerebral venous sinus thrombosis in women that received that vaccine.  Since the cases are all occurring among women from 18 to 50 years old, the rate is more like 1/100,000 for that demographic.

Their diagnosis occurred 6 to 13 days after their vaccinations. Symptoms of this kind of blood clot include severe headache, abdominal pain, leg pain, or shortness of breath- but mostly severe headache.

The treatment for this kind of blood clot is different from what’s typically given for other kinds of blood clots. Usually, an anticoagulant drug called heparin is used to treat blood clots. In the 6 cases that were observed (called cerebral venous sinus thrombosis) heparin is dangerous and patients need a different kind of treatment.

The CDC & FDA recommended that state and county health departments suspend the administration of the J&J shot last Monday. The vaccine is still under emergency use authorization by the FDA. Last week’s advisory was just a recommendation.

CDC held their Advisory Committee on Immunization Practices (ACIP) last Wednesday to review the 6 cases last Wednesday. The ACIP’s job is to assess the potential significance of the adverse events and consider the benefits of the vaccine and make recommendations to CDC and FDA about how CDC and FDA should proceed.

The ACIP’s review examined the details of the cases and concluded that the number of cases is likely above what would normally be expected in that number of people and that the cases are likely related to administration of the J&J vaccine.

I listened to the entire meeting and to be honest I was disappointed at the lack of interest that the committee’s showed about what the public health impacts would be from an extended removal of the J&J vaccine. They seemed to believe that the J&J vaccine is interchangeable with the mRNA vaccines. It is not. 

They failed to appreciate that the J&J vaccine is far more flexible, has an easier cold chain, and is a one dose shot…  making it valuable for getting vaccine to vulnerable and hard to reach populations. They also seemed unconcerned that removal of the vaccine would hurt health equity.

All in all, a disappointing meeting. While they didn’t vote on anything, they appeared to be poised to recommend at least a one month suspension of this important and safe shot. Not a good call.

The ACIP will be meeting again this Friday at 8am AZ time. I hope public health stakeholders like ASTHO and NACCHO reach out to better educate ACIP voting members about the public health costs of a continued suspension of the administration of the J&J vaccine. 

P.S. Keep in mind that ACIP’s recommendations are just advisory and the CDC & FDA Director and Commissioner (or state and county health departments) don’t have to take their advice. Also, FDA does not appear to be considering revoking J&J’s Emergency Use Authorization.

Editorial Note: Anytime you decide to suspend administration of a vaccine like this you need to consider both the costs (risks) and benefits of the decision. One cost is that hundreds of vaccine clinics and events that are prepped and ready to administer the J&J vaccine now have to suspend their operations.

Another risk is that the tens of thousands of persons that would have been vaccinated with the J&J shot in the coming days (weeks) won’t receive their vaccine. Some folks may not come back and may get infected later. The J&J vaccine is uniquely positioned to reach vulnerable populations because of its flexibility.

If the FDA hadn’t temporarily suspend use of the vaccine there’s a risk that additional cases of the rare clot will occur without clinicians having had an opportunity to learn more about the unique treatment for these rare clots.  

The bottom line is that I agree with the CDC and FDA’s decision to temporarily suspend use of the J&J vaccine pending the deeper analysis of these cases last week. However, I was disappointed with the ACIP’s lack of interest in how removal of the J&J vaccine might injure efforts at vaccine access equity.

Dr. Gerald’s Weekly Epidemiology Report

Below is an excerpt from Dr. Joe Gerald’s weekly epidemiology and hospital capacity report. As always, see the full report for all the discussion and informative graphs and analysis.

For the week ending April 4th, 4281 Covid-19 cases were diagnosed in Arizona. This represents a 2% increase from last week’s initial tally of 4200 cases and marks the second week of slightly increasing case rates. The prior week’s tally was upwardly revised by <1% (29 cases) to 4229 cases this week.

Case rates among those ≥65 years of age have fallen below those of children for the first time, 28 versus 37 cases per 100K residents per week, respectfully.

The highest rates remain among those 15 – 24 years and 25 – 64 years of age, 95 and 70 cases per 100K residents per week, respectfully. Arizona case rates remain “stuck” just above the threshold differentiating substantial and moderate risk.

As predicted in the March 26 Update, our 3-month streak of sustained improvements has stalled owing to more transmissible variants (e.g., B.1.1.7) and/or normalization of business and social activities (e.g., EO 2021-06).

Nevertheless, hospital capacity remains adequate to meet Arizona’s needs even though the backlog of non-Covid care has yet to be fully addressed. While residents and businesses should continue to follow public health recommendations, normalization of low-risk activities remains reasonable.

New cases are now being diagnosed at a rate of 59 per 100K residents per week; this rate is neither increasing nor decreasing. For reference, September 8th marked the fall nadir between the summer and winter outbreaks at 38 cases per 100K residents per week.

Post-holiday case rates reached a low of 53 cases per 100K residents on March 23, 2021. Unvaccinated Arizonans who are at risk of developing severe disease (e.g., age or comorbid conditions) or who simply wish to remain uninfected should continue to shelter as much as feasible because viral activity remains widespread.

AzPHA Statement Clarifying Deceptive ADHS Leadership Remarks About Vaccine Administration Equity in Arizona

Director Christ and her communications assistant Steve Elliott have been making statements in the media recently suggesting that Arizona is doing terrific when it comes to the equity of vaccine administration in Arizona.

They have been basing their statements on this CDC MMWR that found AZ had a high level of COVID vaccine coverage in Arizona counties that have a high social vulnerability index. The method used by the CDC compared COVID vaccination rates in counties with high and low social vulnerability indices.

The report did not examine vaccine coverage within counties. CDC’s study design was not designed to detect equity differences inside counties with urban areas like Maricopa and Pima counties (where more than 80% of Arizonans live).

The conclusions in the CDC report that AZ performs well reflect the fact that tribal nations, governments, and the IHS have done a good job vaccinating Native Americans that live in counties  with large numbers of Native Americans (and a high social vulnerability index).

The CDC report cannot and should not be used to draw a conclusion that Arizona is doing well vaccinating lower income and persons of color in Arizona. We are not.

For example, even though 32% of Arizonans are of Hispanic descent, only about 11% of all the persons of Hispanic ethnicity have been vaccinated so far in Arizona. Only 6% of the persons that have been vaccinated at the “state run” PODS have been Hispanic.

The county health departments are working hard to address this disparity with community-based vaccination events. We believe that county health departments should be given a much larger share of the allocated vaccine because their equity performance is much better than that of the “state run” PODS.

Sadly, Director Christ does not see it that way, as she continues to over-allocate vaccine to the stadium-type mega sites at the expense of community-based vaccine sites. We urge her to change this practice, although we do not expect her to do so.