Fear of Vaccine Threatens to Delay Pandemic’s End

After the first vaccines against Covid-19 demonstrated efficacy, public health officials began to foresee a gradual end to the pandemic. Getting there with a minimum of additional suffering will require widespread administration of vaccines, with the goal of achieving so-called herd immunity. Some health specialists warn that vaccine hesitancy -- a reluctance to accept immunization -- threatens to undermine that target.

1. What is herd immunity?

When a large portion of a community (“the herd”) becomes immune to a disease after falling ill or being vaccinated, the pathogen’s transmission from person to person slows and eventually stops. The pathogen dies out within the community. As a result, the whole group becomes protected, not just those who are immune.

2. How much hesitancy exists around Covid vaccines?

In a survey in June across 19 countries, most with a high incidence of Covid, 72% of people said they would take a vaccine that proved safe and effective. Researchers discovered a wide range of acceptance rates among countries, from 55% in Russia to 89% in China. In an article in Nature Medicine, the researchers wrote that in most of the countries, there was insufficient willingness to accept a Covid vaccine to meet the requirements for herd immunity. Since they conducted their poll, national surveys have shown percentages moving up and down. Now that a number of vaccines are being deployed, the U.S. and Europe are seeing relatively high rates of hesitancy among the health-care workers who are among the first to get access to them.

3. What are the requirements for herd immunity?

There’s a fair bit of math and considerable uncertainty involved. The percentage of people who must be immune depends on how contagious a pathogen is. In a November paper in the medical journal The Lancet, researchers calculated that a vaccine that provides lifelong, 100% protection against the coronavirus would need to reach 60% to 72% of people. Two authorized Covid vaccines, one from Pfizer Inc. and BioNTech SE and another from Moderna Inc., were about 95% effective at preventing people from getting sick with Covid, but data isn’t yet available on how effective they were at blocking low-level infections and the risk of passing the virus on. If the vaccines prove to be 80% effective at preventing infection, 75% to 90% of people would need to be immunized, according to the Lancet paper. Another unknown is how long the protection lasts. The shorter the duration, the higher the rates of immunization required to establish herd immunity.

4. What about people who have immunity because they’ve been infected?

It’s possible that a vaccine might not have to do all the work to get to herd immunity. Some people will have immunity because they’ve already had the virus. A study in September concluded that about 9% of people in the U.S. had antibodies to the virus and so, presumably, some level of protection. It’s not known how long this protection lasts and how that compares to the power and duration of immunity provided by Covid vaccines. A December study in the New England Journal of Medicine found that protection after recovering from coronavirus lasts for at least six months except in rare cases; within that period, fewer than 1% of those with antibodies suggesting they were previously infected subsequently tested positive for the pathogen.

5. What if herd immunity isn’t achieved?

One scenario is that some places might get to herd immunity and thus eliminate transmission of the virus, while in other places it would become endemic -- that is, present at all times at a constant level without explosive growth. David Heymann, chair of the World Health Organization’s strategic and technical advisory group for infectious hazards, warned at year’s end that “it appears the destiny of SARS-CoV-2 is to become endemic.”

6. What explains hesitancy around Covid vaccines?

A number of reasons. Some people don’t think the threat of Covid justifies getting a vaccine, or they think the protection they’d get from an actual infection would be superior. The unprecedented speed with which the shots were developed and granted emergency authorization spurred questions about whether they could be harmful. Some people want others to go first and assume the risk. There’s also concern that the process was influenced by political leaders, jeopardizing its integrity. Finally, there are the efforts of anti-vaccination campaigners, who have spread disinformation about Covid and the vaccine effort from the start.

7. What are the risks of new vaccines?

Before regulatory authorities authorize a new vaccine, it must be tested both for safety and efficacy in thousands of human volunteers. Still, there have been cases where safety issues have arisen after authorization. European regulators in 2011 recommended restricting the use of a new swine-flu vaccine from GlaxoSmithKline Plc after it was linked to rare cases of narcolepsy. A vaccine against Lyme disease developed by the same company, then called SmithKline Beecham, was pulled in 2002 amid concerns about links to arthritis. Some vaccines have been shown to do the opposite of what they’re designed to do by inducing unwanted immune responses. In recent years, Sanofi’s vaccine against dengue, which can infect a person as many as four times, was found to cause more severe disease in those who become infected for the first time after getting the inoculation. Among the first 1.9 million people to receive the Pfizer-BioNTech Covid vaccine in the U.S., 21 experienced a severe allergic reaction. None died.

8. How are such cases monitored?

In the U.S., health workers are observing everyone who receives a Covid vaccine for at least 15 minutes to watch for signs of a reaction; those with a worrying history of allergic reaction are monitored for twice as long. Most advanced countries have established systems for reporting adverse side effects of vaccines. In the U.S., anyone can submit a report to the Vaccine Adverse Event Reporting System, which serves as an early warning system to identify side effects. The U.K. has a similar program, called the Yellow Card Scheme. In addition, U.S. authorities have created a system using text messaging and web surveys to check in with Covid vaccine recipients. Vaccine manufacturers are establishing their own means to monitor side effects. Pfizer-BioNTech has announced plans for three studies following people post-vaccination for 30 months.

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