How Much of the Herd Has Some Immunity?

How Much of the Herd Has Some Immunity?

Friday, February 26, 2021 | by Alex Hartzman

Tags: COVID-19, HerdImmunity, pandemic, PHE, Public Health Emergency

COVID-19 cases are currently on the decline in the U.S. and effective vaccines have been developed, approved and are being distributed at record pace – all cause for celebration. However, new variants and societal fatigue are continuing dangers to the health and well-being of Americans as the government and healthcare industry race to vaccinate and protect people from the novel pathogen. As we complete our first full year living in the COVID-19 public health emergency, two questions are worth asking:

  1. How far are we into this pandemic (in terms of herd immunity); and
  2. What are our policy options to take advantage of this recent decline in cases and steadily increasing availability of vaccines?

How far are we into this pandemic (in terms of herd immunity)?

Leading indicators are looking good! First, overall U.S. test positivity rate is below 5%. Test positivity is the portion of tests applied that have a positive result. Positivity rates should be at or below 5% if testing sampling strategies are sufficient and consistent; high and increasing rates indicate potential pandemic growth.

Further, other leading indicators are showing signs that the decline in cases is a real and continuing trend. I use rates of change in new deaths, hospitalizations, and identified cases to help ascertain whether the pandemic is in a growth, decline or transitional phase. The rate of change tells you whether cases are increasing or decreasing and the multiple measures hedge against one another (cases are sensitive to testing strategy, hospitalizations lag infections but are more accurate, deaths lag hospitalizations and are about as accurate as hospitalizations). Rates of change below zero indicate declining infection rates and trends of rate changes are indicative of pandemic acceleration/deceleration (and hence are a window into pandemic phase). Here, I applied substantial smoothing to make a readable chart (i.e., change in deaths is number of new COVID-19 deaths this month divided by deaths last month); we use more sensitive weekly measures for analytic projects.

The progress of COVID-19 from a novel pandemic to an endemic disease entails that much of the novelty (to our immune systems) will be worn off, either through the introduction of the virus to our systems by catching COVID-19 and recovering or through vaccination. This is because infection generally carries with it some amount of protective immunity and states are making progress on distributing vaccines. Below I estimate the percent of the U.S. population with partial immunity due to a prior infection or vaccination to determine our progress to herd immunity. 

COVID-19 infections confer some level of immunity

Inoculation through infection generally carries with it some amount of protective immunity, reducing transmissibility and severity of disease if or when the host is subsequently reinfected. However, the actual number of COVID-19 infections that have occurred in the U.S. is not well-known, and estimates vary widely. First, many if not most people with a COVID-19 infection do not seek testing, test quality and the mix of available tests vary widely, and a subset of people may utilize a disproportionately high number of tests. Further, many people who are infected and are transmissible do not develop symptoms. As such, we must draw cautious estimates of the actual number of infected individuals based on available test data as well as death and hospitalization records, and studies that estimate the expected population effect.

For example, the following chart shows the number of newly identified positive cases per day and the imputed number backcasted from mortality rate estimates (i.e., mortality tells you about the expected case incidence from roughly 3 weeks prior). The number of backcasted cases dwarfs identified positive cases and the ratio of the difference can tell you about the adequacy of testing at any given point in time. The first wave was much more significant than case counts show, probably due to lack of available testing at the time.

States Make Progress on Vaccine Distribution

While use of the newly approved mRNA vaccines may not be inoculation in the traditional sense, it serves the same purpose – protection by enabling the body’s immune system to recognize the pathogen. The reported number of U.S. persons who have been vaccinated is a little more straightforward than counting infections. Major disease reservoirs (e.g., long-term care facilities) and vectors (e.g., healthcare professionals) are the first target for vaccination with the most effective available vaccines, which may be contributing to the current declines in new cases. However, some if not many of those receiving the vaccine may have already been exposed to COVID-19 and have already had partial immunity.

States are making progress on distributing vaccinations, with about 4.6% of the population having received the complete two-dose regimen and another 7.1% having received a single dose (as of February 19, 2021).

Estimating the Portion of Population with Partial or Full Immunity from COVID-19 Infection

To understand the portion of the population who have partial or major protection from COVID-19 from infection and/or transmissibility, I took a range of estimates of the portion of each state’s population (using the reported deaths, and backcasted case counts), and the number of partially or fully vaccinated people. Of course, people who may be vaccinated may already have contracted the disease, so the actual portion of recovered and/or vaccinated may range from low (assuming complete mistargeting of vaccines, e.g., vaccines only delivered to the previously infected) to high (assuming completely perfect targeting of vaccines to those without prior inoculation).

Overall, I estimate 33-45% of the U.S. population has been infected with COVID-19 and/or received at least a partial vaccination. This is a huge portion of the population that is potentially less susceptible to the predominant community COVID-19 strains – we are one third to halfway to herd immunity and transitioning COVID-19 to an endemic disease state.

What are Our Policy Options to Take Advantage of This Recent Decline in Cases and Steadily Increasing Availability of Vaccines?

With less than half the population infected and recovered or vaccinated, public health measures are still needed to prevent another 500,000 or more U.S. deaths. The vaccine race alone is probably not sufficient to head off the next wave of infections and U.S. policymakers are not interested in strict lockdowns because social safety nets are inadequate to handle the resulting mass joblessness in service, hospitality, and entertainment sectors. While school reopenings would probably be safe for many students and greatly support the U.S. economy (including alleviating harms that disproportionately forced women from participating in the workforce), these reopenings are probably not as safe for teachers. That said, the large population of persons gaining immunity through infection and/or vaccination may still interrupt the spread and affect transmission dynamics.

At this point, policymakers can still take significant actions to reduce morbidity and mortality by reducing the size of the next pandemic wave. Now is not the time to back off because things look good temporarily – it is the time to emphasize public health measures to keep the virus in check while immunizations work their way to the U.S. population. Masking, social distancing, indoor gathering and dining bans, working remotely when possible, and otherwise reducing contact to as few people at as much distance as feasible and contact tracing are the commonsense ways of reducing the spread of a highly transmissible air-borne pathogen.

  • Policymakers can let people take personal responsibility for reducing virus transmission by vastly expanding contact tracing programs to enable transmissible persons to self-quarantine for the entire time they are infectious.
  • Policymakers could support their constituents’ ability to prevent infection and reduce community spread by continuing or enacting as strict lockdowns as politically feasible until herd immunity is achieved, even during periods of declining case counts such as now and continue to support mask-wearing and social distancing.
  • Policymaker adoption of mass-vaccination strategies to maximize the speed at which particularly vulnerable populations may be affected should reduce community transmission as more people are vaccinated, even before herd immunity is achieved.

The most effective way to fight this outbreak while the vaccination campaign is ongoing is to strengthen ongoing public health measures and vastly increase contact tracing to encourage people who are exposed to remove themselves from infection-spreading activities before exposing others, reducing the transmission of the disease. Continuing and strengthening these measures until COVID-19 is below local detection thresholds is the surest way to prevent another half million American deaths.

Acknowledgement: Many thanks to Lorenza Jones for research support and Al Dobson, Joan DaVanzo, Kimberly Rhodes and Sandra Agik for editorial review.


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