Lessons learned

Study on Israel’s “silent" polio outbreak explains why people actually get vaccinated

The decision to vaccinate affects everyone.

Scientists are moving at warp speed toward the same goal: creating an effective Covid-19 vaccine. But just manufacturing an effective vaccine may not be enough to curb Covid-19. We'll have to market it effectively.

To create herd immunity and stop the spread of Covid-19, the vast majority of the population must become immune to the disease. This can happen either through vaccination, or by fighting and recovering from the disease. Although the vast majority of people are pro-vaccine, mistrust, fear, and misinformation persist — fueling anti-vax movements that could hinder high Covid-19 vaccine compliance.

New research on the 2013 “silent” polio outbreak in Israel hints how public health officials might market a Covid-19 vaccine effectively.

In the study, published Monday in the journal Proceedings of the National Academy of Sciences, concern for others — or prosociality — led parents to vaccinate their children, more so than their own self-interest.

"As the public faces a potential decision around a Covid-19 vaccine, they should keep in mind that the decision they make not only affects themselves but others as well," first author Chad Wells, a researcher at the Yale School of Public Health, tells Inverse.

This is especially true since Covid-19 poses higher risks for older populations, Wells adds. He notes that it's important for public health authorities to continue to provide reliable information about the "true risks of both disease, and when available, a vaccine."

"This is particularly important so that concerns about vaccine safety don't override prosocial motivations," Wells explains.

In 1988 after a widespread vaccination campaign, Israel was declared polio-free. In 2005, the Israel Ministry of Health advised children to take an inactivated polio vaccine (IPV) instead of an oral polio vaccine (OPV). Both are highly effective against poliovirus, but in very rare cases (about one in 1 million first doses) OPV can lead to paralysis if not given after IPV doses.

While polio was considered effectively eliminated across the country, routine sewage surveillance detected polio in 2013. This suggested the disease was, once again, silently spreading through communities.

To get the outbreak under control, the Israel Ministry of Health proposed a campaign to vaccinate children under 10 years old with the oral polio vaccine (OPV) on top of previously receiving the inactivated polio vaccine.

This presented a unique choice to parents: Children were already protected from paralysis due to IPV, and therefore had no “true” self-interest in vaccinating with OPV. However, secondary vaccination would help stop active community transmission.

Campaigns around the vaccine aimed to appeal to people's concern for others.

“Just two drops, and the family is protected from the risk of polio,” campaign materials broadcasted.

And it worked: Within the first few weeks of the campaign, 79 percent of the targeted population sought out and received a single dose of oral polio vaccine. Over the next two years, environmental surveillance showed polio spread declining and a successful containment effort. On April 28, 2015, the WHO officially declared Israel as a polio-free country.

This outbreak presented an opportunity for Wells and his team to untangle the complicated factors driving parents to vaccinate their children or opt-out. They wanted to know if these vaccinations were out of self-interest — parents were concerned for the health of their family — or if they were driven by a desire to help the community at large.

The team examined survey data from 1,015 parents of children under 10 years old, which captured their reasons for vaccinating their offspring. The researchers broke down the participants by their level of awareness and their motivations.

Parents were classified as aware or unaware of the prosocial nature of the campaign and those who were aware were then classified as prosocial or individualistic. Individualists were acting only to maximize payoff for their family, the researchers say.

"In Israel’s silent polio outbreak, prosocial and self-interested motives were operating in combination. For example, some parents were vaccinating not only to protect their child but others as well," Wells says.

Forty-five percent of the Israeli population were unaware, 17 percent were individualistic, and 38 percent were prosocial. People who acted in self-interest said they hoped to protect their child or family. Sixteen percent of parents who chose to skip OPV cited vaccine safety as their main reason.

Then, the researchers created unique game theory models to identify and quantify the drivers of vaccination coverage.

Classic game theory suggests people vaccinate their children or receive vaccinations as an act of self- interest. They either decide to protect themselves through vaccination or take a “free ride” on the population-level protection built up by others, the researchers write.

But acting purely in self-interest and taking a "free ride" can also prevent communities from reaching herd immunity and truly eliminating infectious disease. This team added in prosocial motivations, on top of self-interest in the models.

After analyzing the data and applying game theory, researchers found that prosocial behavior was a key driver of OPV vaccination during the 2013 outbreak. In fact, prosociality was more likely to influence vaccination behavior than self-interest.

The team found that the prosocial model was 17 times more likely than the individualistic model to achieve high vaccination coverage, over 70 percent, as reported by the survey and the Israel Ministry of Health.

"Prosocial behavior, which is people acting for the benefit of others, was critical for Israel to achieve its high vaccination coverage during the 2013 oral polio vaccination campaign," Wells explains.

But at the same time, increasing vaccination coverage through promoting prosocial behavior or concern for others is an "exceedingly difficult task," Wells explains.

Interestingly, people who understood the campaign's prosocial nature were likely to have a higher perceived risk of the relative risk of paralysis (vaccine versus infection) compared to those who did not understand the nature of the campaign.

"Communication should also be directed towards alleviating misinformed fears and risks about the vaccine," Wells says.

The country-wide case study offers lessons to the current, global pandemic, the research team says. When it comes to a Covid-19, it's possible that prosocial and self-interest motivations will both drive vaccination acceptance.

It's possible vaccine campaigns may need to encourage younger people to vaccinate not to protect themselves, but the elderly. Meanwhile, because Covid-19 can cause severe complications, young people may be motivated to get vaccinated to protect their personal health.

While scientists test vaccine candidates, it's worthwhile to think carefully about how they will be presented to the public.

"Promoting behavior change, debunking misinformation, and mitigating fears around vaccines and other measures is an extremely complex issue," Well says. "There likely is not a single fit-all solution."

Abstract: Regions with insufficient vaccination have hindered worldwide poliomyelitis eradication, as they are vulnerable to sporadic outbreaks through reintroduction of the disease. Despite Israel’s having been declared polio-free in 1988, a routine sewage surveillance program detected polio in 2013. To curtail transmission, the Israel Ministry of Health launched a vaccine campaign to vaccinate children—who had only received the inactivated polio vaccine— with the oral polio vaccine (OPV). Determining the degree of pro- social motivation in vaccination behavior is challenging because vaccination typically provides direct benefits to the individual as well as indirect benefits to the community by curtailing transmission. However, the Israel OPV campaign provides a unique and excellent opportunity to quantify and model prosocial vaccination as its primary objective was to avert transmission. Using primary survey data and a game-theoretical model, we examine and quantify prosocial behavior during the OPV campaign. We found that the observed vaccination behavior in the Israeli OPV campaign is attributable to prosocial behavior and heterogeneous perceived risk of paralysis based on the individual’s comprehension of the prosocial nature of the campaign. We also found that the benefit of increasing comprehension of the prosocial nature of the campaign would be limited if even 24% of the population acts primarily from self-interest, as greater vaccination coverage provides no personal utility to them. Our results suggest that to improve coverage, communication efforts should also focus on alleviating perceived fears surrounding the vaccine.
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