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Will we Have Enough Vaccine to Reach Herd Immunity?

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While there are several significant hurdles to achieving herd immunity, vaccine production pledges for the US for our three currently available vaccines include more than enough vaccine to vaccinate every adult in the country by the summer. Pfizer and Moderna have pledged to provide 600 million doses, enough to provide 300 million people with full immunization, and J&J has pledged an additional 100 million doses of its single shot vaccine.

Despite anticipated increases in vaccine supply, a recent Nature article highlighted 5 reasons why achieving herd immunity will be challenging, adapted here:

#1: It’s unclear whether vaccines prevent transmission (though it seems that they do)

More definitive data is coming out on this point, making this seem like the least of our challenges to herd immunity.  Researchers reported March 29th in Morbidity and Mortality Weekly Report on a study led by CDC researchers collecting weekly nasal swabs for coronavirus testing from December 14th through March 13th from nearly 4,000 vaccinated health care workers, first responders and other essential workers in six states – both symptomatic and asymptomatic infection rates fell dramatically two weeks after the first vaccination, contributing to the growing body of evidence suggesting that vaccines not only reduce the risk of getting seriously ill with COVID-19 but can also prevent transmission.

#2: Vaccine roll-out is uneven

While a perfectly coordinated global vaccination campaign could have wiped out COVID-19, there are huge variations in the efficiency of vaccine roll-outs between and within countries.  “No community is an island, and the landscape of immunity that surrounds a community really matters,” says Dr. Shweta Bansal, a mathematical biologist at Georgetown University.  COVID-19 has occurred in clusters across the United States as a result of people’s behavior or local policies. Previous vaccination efforts suggest that uptake will tend to cluster geographically, too, Bansal adds. Localized resistance to the measles vaccination, for example, has resulted in small pockets of disease resurgence. “Geographic clustering is going to make the path to herd immunity a lot less of a straight line, and essentially means we’ll be playing a game of whack-a-mole with COVID outbreaks.” Even for a country with high vaccination rates, such as Israel, if surrounding countries haven’t done the same and populations are able to mix, the potential for new outbreaks remains.

#3: New Variants

Even as vaccine roll-out plans face distribution and allocation hurdles, new variants of SARS-CoV-2 are sprouting up that might be more transmissible and resistant to vaccines. The longer it takes to stem transmission of the virus, the more time these variants have to emerge and spread.  Higher rates of immunity can create selective pressure, which would favor variants that are able to infect people who have been immunized. Vaccinating quickly and thoroughly can prevent a new variant from gaining a foothold. But again, the unevenness of vaccine roll-outs creates a challenge.

#4: Immunity might not last forever

Calculations for herd immunity consider two sources of individual immunity — vaccines and natural infection. People who have been infected with SARS-CoV-2 seem to develop some immunity to the virus, but how long that lasts remains a question. And we’ll have to account for the fact that the vaccines are not 100% effective. If infection-based immunity lasts only for something like months, that provides a tight deadline for delivering vaccines. It will also be important to understand how long vaccine-based immunity lasts, and whether boosters are necessary over time. 

#5: Vaccines might change human behavior

The problem is that, as more people are vaccinated, they will increase their interactions, and that changes the herd-immunity equation, which relies in part on how many people are being exposed to the virus. The most challenging aspects of modeling COVID-19 are the sociological components, says epidemiologist Lauren Ancel Meyers, executive director of the University of Texas at Austin COVID-19 Modeling Consortium. “What we know about human behavior up until now is really thrown out of the window because we are living in unprecedented times and behaving in unprecedented ways.” Meyers and others are trying to adjust their models on the fly to account for shifts in behaviors such as mask wearing and social distancing.To understand the additive effects of behavior and immunity, consider that this flu season has been unusually mild. “Influenza is probably not less transmissible than COVID-19,” says Samuel Scarpino, a network scientist who studies infectious diseases at Northeastern University. “Almost certainly, the reason why flu did not show up this year is because we typically have about 30% of the population immune because they’ve been infected in previous years, and you get vaccination covering maybe another 30%. So you’re probably sitting at 60% or so immune.” Add mask wearing and social distancing, and “the flu just can’t make it”, Scarpino says. This back-of-the-envelope calculation shows how behavior can change the equation, and why more people would need to be immunized to attain herd immunity as people stop practicing behaviors such as social distancing.

The scientists interviewed for the article emphasized that these concerns for herd immunity should not overshadow the fact that vaccination is helping.  While long-term prospects may include COVID-19 becoming an endemic disease, much like influenza, this isn’t as grim as it may sound.  Even without herd immunity, the ability to vaccinate vulnerable people seems to be reducing hospitalizations and deaths from COVID-19.  The disease might not disappear any time soon, but its prominence is likely to wane.

Jennifer Abrams, MD, March 30, 2021