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COVID-19: Are we seeing ‘exponential decay’?

Health and Benefits|Wellbeing
COVID 19 Coronavirus

By Jeff Levin-Scherz, MD | May 4, 2021

Mass vaccination is clearly having an impact on the spread of COVID-19.

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About our “The COVID-19 Crisis” series

“The COVID-19 Crisis” series is a weekly update by Dr. Jeff Levin-Scherz covering the latest developments related to the COVID-19 pandemic in the U.S. Explore the entire blog series.

COVID-19 news in the U.S. keeps on getting better. Our seven-day average of new cases was 53,500 on April 28, down about 25% over the past two weeks. We continue to have over 37,000 hospitalized, about the same as in mid-March. New hospitalizations per day, about 5,000, remain similar to early March as well. I’d expect hospitalizations to go down over the next month as they lag new cases substantially.

In the U.S. over the past two weeks, the number of new COVID-19 cases have been declining. - Description below

The seven-day moving average on May 1, 202,1 was 48,860. This number was almost 70,000 on April 14.

Number of new COVID-19 cases in the U.S. reported to the CDC

Seven-day moving averages through May 1, 2021.
Source: CDC

Even cases in Michigan are down more than 42% over the last two weeks.

On the vaccination front, almost 100 million Americans have been fully vaccinated, and nearly 144 million have had at least one injection. We are at an average of 2.6 million daily doses — down from the peak three weeks ago, but still a vaccination rate that will continue to drive down the rate of new cases.

During surges we’ve heard a lot about the “exponential growth” of COVID-19 infections — which can lead to overwhelmed hospitals and widespread death. What we hope we are seeing now is the phenomenon of exponential decay, where, on average, each new case spreads to fewer and fewer others. That’s how pandemics sputter out.

Exponential growth is what we’re seeing now in India, where there are more than 300,000 new cases reported each day, and the actual number is likely many times higher. Exponential decay is what we’re seeing in Israel, where a rapid vaccination program led to dramatic declines in COVID-19 cases and allowed the economy to fully reopen.

The Centers for Disease Control and Prevention (CDC) reported on breakthrough cases of COVID-19 in those who were fully vaccinated. These cases are prominently covered by the press, but the COVID-19 vaccines remain highly effective in the “real world” setting — despite growth of some worrisome variants. One exception is CoronaVac (a Chinese vaccine that has not released data from its Phase 3 testing). It appears far less successful, especially in South America, which is suffering a terrible surge now.

Risk in fully vaccinated individuals

Data from the CDC based on the first 87 million who were fully vaccinated. These risks will likely go up somewhat with longer observation. But it’s likely that the risk will remain small.
Post-vaccination risks Number of occurrences Probability
Infected with COVID-19 7,157 cases 1 in 12,000
Hospitalized with COVID-19 331 hospitalizations 1 in 250,000
Died of COVID-19 77 deaths 1 in 1,000,000

New mask guidelines from the CDC

The CDC announced new recommendations around masking this week. There are large distinctions between those who are vaccinated and those who are not. Vaccinated people are no longer recommended to wear masks outdoors, except in large crowds. Indoor dining and gyms are now considered acceptable for those who are vaccinated (with a mask), though both remain higher risk because of the length of time people spend in such places.

The CDC continues to recommend outdoor masking for those who are not vaccinated, although the likelihood of transmission of COVID-19 outdoors is tiny unless one is stationary, talking in close proximity to others for extended time periods. We’re likely to see changes in these recommendations over the coming weeks as community transmission rates decline.

You might have read news reports that millions have skipped their second dose of vaccine. The CDC said that 92% of people who had gotten a first vaccine got their second shot within the recommended window. That’s a staggering success. Historically, only two-thirds of adults generally return for the second shot of a two-injection vaccination regimen. The first Pfizer or Moderna shot offers some protection (after 12 to 14 days), but the protection two weeks after the second shot is dramatically greater and is likely to last much longer. We should strongly encourage all to get their second shots (or get the one-dose Johnson & Johnson vaccine), but I found this report heartening rather than worrying.

A hundred colleges and universities have announced mandatory vaccination, which is consistent with near-universal requirements that college students are immunized against measles to prevent campus outbreaks. Most of these schools require vaccination of students, but not faculty or staff.

We’ll continue to see more requirements to be vaccinated for entry into entertainment or other venues, or see those who are vaccinated be able to skip a mandatory real-time test — much as those with TSA Precheck don’t have to remove their shoes or their laptops before airport screening. Few employers have issued mandates at this point, although some commentators advocate a vaccine mandate for the highest risk employees (such as first responders and those who work at nursing homes, hospitals, prisons and meat packing plants).

As more Americans receive vaccinations, hospitalizations and deaths will likely continue to decline despite the rising prevalence of more contagious variants. We can see a return to something much more like normal even before we reach herd immunity.


Population Health Leader, Health and Benefits, North America

Jeff is a practicing physician and has led Willis Towers Watson’s clinical response to COVID-19. He has served in leadership roles in provider organizations and a health plan, and is an Assistant Professor at Harvard Chan School of Public Health.

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