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 using vaccines to achieve so-called herd immunity. Some health specialists warn that vaccine hesitancy -- a reluctance to accept immunization -- threatens to undermine that goal.

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. As a result, the whole community becomes protected, not just those who are immune.

2. How much hesitancy exists around a Covid vaccine?

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. Survey results have varied since they conducted their poll. But in a survey in the U.S. in November, just 60% of people said they would probably or definitely get a Covid vaccine. Across seven European countries, 57% of people polled the same month were willing.

3. What are the requirements for herd immunity?

There’s a fair bit of math and some uncertainty involved. The percentage of people who must be immune depends on how contagious a pathogen is; for the coronavirus that causes Covid, a November paper in the medical journal The Lancet put the range at 60% to 72%. To achieve the lower figure with a vaccine that’s 95% effective, like the one from Pfizer Inc. and BioNTech SE that the U.K. began deploying Dec. 8, requires immunizing about 63% of people. Using the vaccine developed by the University of Oxford and AstraZeneca Plc, which was 70% effective on average, would require inoculating 86% of people. However, a vaccine might not have to do all the work; some percentage of people should already be immune because they’ve recently 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 against it.

4. What explains the hesitancy?

A number of reasons. Some people don’t think the threat of Covid justifies getting a vaccine. The unprecedented speed with which Covid vaccines were developed and reviewed raises questions about whether there are safety issues that haven’t emerged yet, leading some people to 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.

5. What do vaccine opponents believe?

While opposition to immunizations has been around for as long as the shots, the most recent anti-vaccine movement took off after the The Lancet published what turned out to be a fraudulent study in 1998 linking the measles, mumps and rubella vaccine to autism. The Lancet retracted the study in 2010 and the U.K.’s General Medical Council stripped its author, Andrew Wakefield, of his medical license for “dishonest” and “irresponsible” work. By then many parents had latched on to the idea that vaccines were to blame for a rise in autism diagnoses. There were subsequent claims that thimerosal, a preservative in some vaccines, caused autism. Both theories have been definitively debunked.

6. 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 people can get as many as four times, was found to cause elevated cases of severe disease for those who’d not previously been infected by the virus and caught it after innoculation. Two people who received the Pfizer-BioNTech Covid vaccine in the U.K. experienced allergic reactions, prompting the National Health Service to advise that people with a significant history of allergies not receive it. Such people were excluded from trials testing the vaccine.

7. How are such cases monitored?

Most advanced countries have established systems for reporting adverse side effect 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. For example, the Pfizer-BioNTech application for emergency authorization in the U.S. includes a plan for three studies following people post-vaccination for 30 months.

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