Will You Need A Booster Vaccine Against Covid-19?

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Infectious disease expert Dr. Mark Kortepeter discusses what we know so far about Covid-19 vaccine booster shots and what information scientists need to make the decision.


When President Joe Biden recommended in August that everyone get a booster eight months after their last dose, it caught some people in the scientific community and the government by surprise.  Though even in announcing the September rollout, there was a catch. Biden acknowledged the booster plan still needed to be OKed by scientists at the FDA and the CDC. The scientists have now spoken. An FDA advisory committee met on September 17th to review the science of the Pfizer vaccine and after much discussion voted in favor of a third vaccine dose only for people aged 65 and over, as well as those considered at high risk. The committee, however, felt that the safety and efficacy data did not support a third dose for everyone at this time. Here’s what we know about boosters.

What is a booster?

Many vaccines typically consist of a primary series and boosters. The primary series includes one or more doses over the course of several months to years to give someone basic protection against a specific bacterial or viral pathogen, which we refer to as “fully vaccinated.” A booster is an additional dose of the vaccine beyond the primary series.

Boosters are not unusual. For example, the primary series of the vaccine against measles virus is given as a single dose around one year old. We didn’t used to give a booster, because the single dose was so effective. It wasn’t until there was a large outbreak of measles in U.S. school children in 1989 that we realized some children didn’t respond to the first dose. Hence, a second dose is now recommended to be given between ages 4-6 years old.

It is generally harder to maintain immunity against bacterial pathogens, so vaccines against bacteria typically require more shots in the primary series. For example, the vaccine against whooping cough (pertussis) requires five doses over several years for the primary series, followed by a booster at age 11-12.

For the Covid vaccines, the ones made by Pfizer and Moderna currently require two doses and the Johnson & Johnson vaccine requires one dose to be considered fully vaccinated. Under the current emergency use authorization, a third dose of the Pfizer vaccine can also be given to immunocompromised individuals.

Why has it taken so long to decide on Covid vaccine boosters?

Usually, a pharmaceutical company has several years to follow people vaccinated in a research study to determine the duration of protection and the optimal time to give boosters. Because of the pandemic emergency, there was a critical need to roll out vaccines before individuals could be followed for months or years to make this determination. This leaves us trying to make vaccine policy in real-time while the pandemic continues. To use a common analogy, we are “building the plane while flying.”

It is possible to follow the levels of antibodies people have in their blood over time. If the antibody levels drop, then it may indicate someone needs a booster. It isn’t so simple, though. Antibodies are only one piece of the puzzle. The human body has other ways it fights viruses, which are harder to measure. We don’t even know what level of antibodies are required to prevent disease and therefore what level we should strive to reach. The best way to learn how long the vaccines work is to get “real world” data on how many people get infected and sick over time.

What do we know so far?

When the Covid vaccines first rolled out, the early data demonstrated that they were over 90% effective at preventing severe disease and death, but we had no idea how long that protection would last. There was early concern that individuals with compromised immune systems would not respond as well to the vaccines. Consequently, there is already a recommendation for them to receive a third dose at least 28 days after the second one.

For everyone else, as Covid continues to circulate around the world and the new highly contagious delta variant tears through the population as fast as the wildfires engulf northern California, we are seeing increasing “breakthrough” infections in those who were fully vaccinated. However, recent data from the CDC comparing two periods of time in 2021 (April 4-June 19 versus June 20-July 17)  indicate that although there have been increasing rates of Covid infection in the vaccinated group over time, protection against severe disease and death has declined only slightly. The decline was greater in those 65 years and older.  ICUs across the U.S. continue to be filled mostly with the unvaccinated. The CDC data only goes through July 2021, so it may not be the full picture of where we are headed. Recent data from Israel show declining protection against disease and severe disease (especially in those greater than 60 years old) six months after receiving the second Pfizer vaccine. Could the Israeli experience be a harbinger for what will happen in the US? Possibly, but we won’t know until we have more time to gather more data.

Who makes the decision on boosters?

Despite the White House’s recently announced intention to give people a Covid booster eight months after the primary series, neither the White House nor another country’s policies, a single scientific paper, or your favorite social media blogger determine when a booster is needed. The way the process works is that the company product sponsor has to collect the “real world” data and present it to the FDA, where scientists and regulators pour over it. The FDA has the regulatory authority to make the final decision on whether a particular vaccine has demonstrated safety and efficacy for it to be given as a booster. It is a very deliberate process, and it needs to be, because it impacts peoples’ health. FDA advisory committees are an important part of the process. The September 17th advisory committee considered multiple sources of data, including from the company (in this case, Pfizer). The committee’s determinations may have answered some questions, but they still may have left many confused. We are seeing scientific debate now in sharper focus than usual, because the stakes are high with the ongoing pandemic. However, disagreement among scientists is common and it is a healthy part of the scientific process.

Will you need a booster?

So, should you get a booster now? If you can envision a staircase, consider the top of the stairs as the highest risk for severe disease. Each step, then, adds a risk factor for more severe disease, for example, age over 65 might be one step, immunocompromising illness might be another step or more. If you have a job that puts you at greater risk of interacting with sick people (for example, a healthcare provider, first responder, nursing home aid, teacher), that could be another step. Another might be a longer duration since your second vaccine dose. When you add up all your risk factors, the higher you find yourself on the staircase, the more reasonable it is for you to get a booster.

Keep in mind that if you have been fully vaccinated, your immunity isn’t like a light switch – on one moment and off the next. If it is going to decline, it does so gradually over time. It would be nice if we had a crystal ball to know what the data in three or six months will show, but we don’t have that. People want answers quickly, but science doesn’t move at the speed of social media. It never will – and that is a good thing.

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