Vaccine Q&A: Herd immunity and prioritizing access to vaccines

sign reading "together we can help stop the spread of covid-19"

In this post, we focus on herd immunity, why some people are unable to receive a vaccine, and how we determine who has priority in getting access to COVID-19 vaccines.

To address those questions, we spoke with Julie Swann, a systems engineer with expertise in vaccine distribution whose work focuses on making health care and supply chains more efficient, effective and equitable. Swann is the department head and A. Doug Allison Distinguished Professor of the Fitts Department of Industrial and Systems Engineering at NC State.

This post is part of a series of Q&As in which NC State experts address questions about the vaccines on issues ranging from safety to manufacturing to how the vaccines will be distributed.

The Abstract: Will you take a COVID-19 vaccine?

Julie Swann: My family looks forward to when the COVID-19 vaccines will be widely available. We know that children were not studied in the clinical trials so far, so I am not ready to vaccinate the youngest among us until more data is available on those populations. Similarly, pregnant women, people with some immunocompromised conditions, and other populations with specific risk factors were not included in the initial trials, so they should speak with their physicians about whether the vaccine is right for them. There may also be differences in some of the vaccines, which could play a role in determining which are most appropriate for a particular person.

But, yes, I plan to take the vaccine that my physician thinks is best for me. Doing so will not only help protect me, but it will also help protect those around me, including my family members and the broader community.

TA: Why are some people unable to take the vaccine?

Swann: The companies that planned the clinical trials for the vaccine intentionally recruited a diverse set of participants, representing different racial or ethnic groups, ages, people living with HIV or Hepatitis B or C. However, there are others who were unable to participate in the trial, such as children, pregnant women and most people with cancer. For people who are immunocompromised, their system may not respond to the vaccine. And people with severe allergies were also not included in the first trial.

Individuals can discuss with their physician whether the vaccine is right for them, balancing their own risks and potential outcomes. Those individuals may need to continue shielding themselves from SARS-CoV-2 whenever possible.

However, there is something else that all of us can do to help those who cannot take the vaccine. If the household members of pregnant women or people with cancer take the COVID-19 vaccine, then it may help provide some protection to that individual*. At a community level, the more of us who are vaccinated against COVID-19, then the more protection there will be for those who cannot take the vaccine – which can be particularly important for people who are susceptible to severe outcomes. This is one of the benefits of “herd immunity.”

[*Note: There is strong evidence the vaccine prevents symptomatic cases of COVID-19, which tend to be more severe and longer-lasting. It may be possible that some of the symptomatic cases are shifted to asymptomatic ones, which can still be transmitted to others. This means that someone who has taken the vaccine may still be able to transmit the disease to others. Masks are encouraged when around people who are vulnerable. You can learn more about the possibility that vaccinated people may still be able to transmit COVID-19 here. This is an ongoing area of research, and we will share new information when it becomes available.]

TA: What is herd immunity?

Swann: Herd immunity has received a lot of attention this year. It is what happens when enough people in the population have protection against a virus, so that the virus spread begins to drop off. This herd protection, as it is also called, does not mean that no one can get the virus anymore. In a large population (like the U.S., or even just N.C.), we would expect the level of protection to vary geographically. In addition, even if the overall herd has some protection, that doesn’t mean that every individual is protected. Herd protection can be achieved with sufficient vaccination rates, where the specific level that is needed varies by the disease.

TA: How many people need to get vaccinated in order to reach herd immunity? And how do we know that?

Swann: The proportion of people who need to be vaccinated to provide resistance to the virus relates to the infectivity of a disease (sometimes measured by the reproductive rate, or R0, which is the average number of people who would be infected from a single case at the start of the pandemic). For example, for measles (which is highly infectious, with an R0 around 12-18), about 19 out of every 20 people need to be vaccinated to have some level of protection in the community.

For SARS-CoV-2, the virus that causes COVID-19 disease, we think the R0 is between two and three, meaning that an individual (on average), would infect two to three other people if there were no interventions in place and no one else had immunity. Given that, 50-67% of the population would need to have immunity before the virus would slow down.

TA: Who ultimately decides which people are prioritized to receive the vaccines? And what factors are taken into consideration when determining when different groups of people have access to the vaccines?

Swann*: It is a little complicated. Maybe more than a little complicated.

The Advisory Committee on Immunization Practices (ACIP) is an advisory committee to the U.S. Centers for Disease Control and Prevention (CDC). It’s ACIP’s job to make recommendations about vaccination priorities to the CDC, and the committee issued interim guidance on how to allocate vaccine supplies on Dec. 11. That interim guidance was fairly broad, simply stating that both health care personnel and residents of long-term care facilities be offered COVID-19 vaccine in the initial phase of the vaccination program.

But one reason I said this subject is complicated is because there are a lot of other players as well.

As vaccines are produced, doses are allocated to jurisdictions (e.g., states, territories and a few large cities) by the federal government. Those jurisdictions then direct the vaccine to specific providers who are tasked with vaccinating critical populations as outlined in each jurisdiction’s distribution plan. States were asked to submit initial plans by mid-October, but the final ACIP recommendations are based on reviewing the safety and efficacy data of a vaccine during the approval process, which happens later. Even before the ACIP recommendations, the NIH and the CDC had sponsored a group from the U.S. National Academies of Sciences, Engineering, and Medicine to develop a consensus report to lay out a framework for prioritization of vaccine. This is fairly similar to what happened during the H1N1 pandemic, where the ACIP recommended priority groups to the CDC, and the execution of the plan was made by state and local health organizations.

The ACIP has determined goals for allocating a limited supply of vaccines, which are derived from several ethical principles they outlined. One key goal is reducing death and severe disease, which matches with allocating vaccine to healthcare workers who are at greater risk of exposure to SARS-CoV-2 and to nursing home residents who are greater risk of dying from an exposure.

A second goal is ensuring that society can function, which relates to prioritizing essential workers who help keep our critical infrastructure operational. As the CDC states on its website, the third and fourth goal are to “reduce the extra burden … on people already facing disparities” and to “increase the chance for everyone to enjoy health and well-being.” In practical terms, these goals mean some groups may receive priority, such as adults with high-risk health conditions, adults 65 and over, minority groups with excess burden from COVID-19, people experiencing homelessness, and people living in rural communities. North Carolina, for example, has a detailed list of higher-risk groups in its state distribution plan, as well as plans to reach them.

The CDC expects that in phase 1 of the vaccine distribution, there will be a limited supply of COVID-19 vaccine, and it is recommended that those doses be allocated to priority populations. In phase 2, a larger number of doses will be available, this is likely when vaccinations will begin to become available to the general population on a limited basis. In phase 3, there will be a sufficient supply of vaccine for the entire population.

[*Editor’s Note: the CDC released more information on Dec. 20, which will likely affect how states prioritize who receives vaccines for COVID-19. We followed up with Julie Swann to update this Q&A.]

Swann: The guidance on vaccine priorities continues to change. Recently, the CDC considered priority groups after frontline medical personnel and long-term care residents. The next groups discussed are people 75 years or older, and frontline essential workers. In that last category, they focused on people at high risk of exposure who are also critical to the function of society. This included firefighters and police, educational workers (including daycare and support staff), food and agriculture industry, manufacturing, corrections personnel, U.S. Postal Service, public transit, and grocery store workers in the next subpopulations. Other essential workers were listed as a priority for future vaccination, including transportation and logistics, food services, housing and construction, finance, information technology and communication, energy, media, legal, public safety engineers, waste and wastewater workers. States will have a large role in determining the priorities from their allocations, and it may differ some in timing and specific industries by geography.

TA: How will people know when they are able to get vaccinated?

Swann: I encourage people to pay attention to your local news, including print, radio, and television. You may also hear from your primary care provider or local pharmacy on which populations are eligible for vaccination at any given time. I expect the allocation and priorities to be dynamic, as the supply of vaccine changes, acceptance of vaccine shifts, and risk areas become more evident. I know I am trying to be patient as I give those most at risk a chance before it is my turn.

This post was originally published in NC State News.