Vaccine Demand is Falling, Why It Matters


Three takeaways from our analysis on possible Herd Immunity Threshold and how to reach it together.

Ugur Koyluoglu, Bruce Hamory, MD, Terry Stone, and Elizabeth Shakhnovich

7 min read

In the urgency to create, approve, and distribute the precious vaccine, many lost sight of the fact that demand – not supply – would ultimately prove to be the key to returning to normalcy in the US. That turning point came seemingly overnight – from endless stories of frantic browser refreshing in the hopes of securing an elusive appointment, to rapidly amplifying news of unused appointments, states declining allocated doses, and vaccination centers closing. In the span of two weeks, the US daily administration average has dropped from 3.3 million to 2.3 million, signaling a dramatic decline in demand despite the fact that only 32 percent of the US population is fully vaccinated. 

The US daily vaccine administration average has dropped from 3.3 million to 2.3 million.

Daily administration data and recent surveys have shown a growing gap between states like Vermont – where demand remains high and daily administration continues to climb – and Mississippi – where little willing population remains and the daily administration has declined significantly past plateau. We used US Census Bureau poll data to update our assumptions on anticipated vaccine uptake by state and examine the impact on timing to pHIT (possible Herd Immunity Threshold) by state across the US. Below, we list three key takeaways from our analysis:

1. While the US, on average, is still on track to achieve pHIT by mid-summer, we see a growing divide between states that are tracking well and others that are at significant risk of not achieving pHIT at all.

As a reminder, pHIT is the point at which there is enough cumulative immunity in the population (as a result of natural infection or vaccination) that we are no longer at risk of exponential growth. We expect pHIT to occur between 67 percent and 75 percent cumulative population immunity, with a point estimate of 71 percent. Transmission does not stop in a population that has achieved pHIT.  Sporadic outbreaks will still occur at this point, but there would be no risk of a wave of cases that could overwhelm our health systems. Some states (such as Massachusetts, New Jersey, New York, and Connecticut) are likely to reach pHIT in the next few weeks, while others (such as Louisiana, Mississippi, and Alabama) are not going to reach pHIT at all if the current trajectory of natural infection and vaccine uptake holds.

2. Every vaccinated person matters.

In many states, even a five percent difference in vaccine uptake can make a dramatic impact on timing to pHIT, changing the timeline by a month or more, or potentially stalling the state just below the threshold. Surveys have shown that the currently unvaccinated are not a homogeneous group – they differ dramatically in the reasons behind their reticence to become vaccinated (for example, lack of computer literacy or physical access, lack of understanding, or general anti-vaccine sentiment) and therefore in the likelihood that they can be convinced to become vaccinated. It is imperative that states – particularly ones at risk of not achieving pHIT – take a detailed look at the unvaccinated population and deploy targeted strategies to combat vaccination barriers.

3. Regardless of the threshold or whether we exceed it, more vaccination is better. 

The question of whether herd immunity threshold exists and at what level of cumulative immunity it can be achieved has been hotly debated since the start of the pandemic. We may soon be able to infer the answer based on real-world evidence from front-running states and countries like Israel and the United Kingdom. But regardless, even if pHIT cannot be reached (at all or in a particular location), we must not give up on cumulative immunity as a goal. We – along with much of the scientific community – anticipate that global and local differences in cumulative immunity levels will allow the virus to become endemic. Under this circumstance, more vaccinated people mean a larger break in the transmission rate of the virus and a smaller reservoir for the emergence of variants. A local geography with a high rate of cumulative immunity will always be safer (for everyone) than an area with less, which means we should not only pursue maximum cumulative population immunity but should continue vaccinations even if we reach the threshold to maintain it on an ongoing basis. Click "Play/Pause" below to view our animated trending progress toward pHIT by state. 

  • Ugur Koyluoglu,
  • Bruce Hamory, MD,
  • Terry Stone, and
  • Elizabeth Shakhnovich