Understanding the second wave of COVID-19 and what comes next

Originally published on December 2, 2020

How dangerous was the second wave of COVID-19, how close were populations to herd immunity, and why were vaccines never going to end the pandemic overnight? In Episode 101 of the WHOOP Podcast, Yale University physician, sociologist, and author Dr. Nicholas Christakis explained the mechanics of asymptomatic spread, the real meaning of fatality rates, the cost of testing failures, and the longer recovery timeline that many people did not want to hear.

This article distills the clearest insights from that discussion, including why SARS-CoV-2 spread so efficiently, what public health systems missed on masking and testing, and what WHOOP data can tell you when respiratory rate shifts away from baseline.

To listen to Episode 101 of the WHOOP Podcast in full, head to the WHOOP Podcast on Spotify.

Why was the second wave of COVID-19 so hard to contain?

SARS-CoV-2 spread efficiently because it combined real lethality with strong transmissibility before symptoms appeared. Christakis contrasted it with SARS-CoV-1 from 2003, which killed faster and therefore burned out more easily.

He argued that a pathogen can become more dangerous at a population level when infected people stay mobile long enough to pass it on. That pattern also helps explain why respiratory rate mattered. In Episode 80 of the WHOOP Podcast, golfer Nick Watney described how a jump from about 14 to 18 breaths per minute in the WHOOP app helped prompt a test before symptoms appeared. Similar patterns later showed up in Episode 71 of the WHOOP Podcast.

As Christakis explained, the key problem was timing:

“Some estimates are that 75% of the cases acquired in a typical population are acquired from other people who are asymptomatic.”

What you should take away

• SARS-CoV-2 was hard to contain because people could spread it before they felt sick.
• A disease that is less lethal per case can still cause more total deaths if it transmits more widely.
• Respiratory changes, including an unusual rise in respiratory rate, can be an early signal that something is off.

If you want to hear Christakis unpack asymptomatic spread and transmission timing, listen to the full episode on Spotify.

How deadly was COVID-19, and what did the fatality rates mean?

Once transmission is clear, the next question is severity. Speaking in December 2020, Christakis said multiple analyses pointed to an infection fatality rate between 0.5% and 0.8%, with the case fatality rate roughly double that because many infections never produced symptoms.

He also stressed the age curve. Younger adults faced much lower absolute risk than older adults, but low risk was not zero risk, and younger people could still seed transmission into more vulnerable groups. His point was practical: a low baseline chance of death is not a reason to accept an avoidable infectious risk.

Christakis put the definitional piece plainly:

“The infection fatality rate of this pathogen is between 0.5 and 0.8 percent.”

What you should take away

• Infection fatality rate refers to deaths among all infected people.
• Case fatality rate refers to deaths among people whose cases come to medical attention or develop symptoms.
• Lower risk in younger people did not remove the social risk of passing the virus to others.

If you want to hear Christakis go deeper on infection fatality rate, case fatality rate, and age risk, listen to the full episode on Spotify.

What went wrong with masks, testing, and the early public health response?

If a virus spreads before symptoms and carries a real fatality burden, speed matters. Christakis argued that the United States had multiple warning shots, from China’s lockdown of 930 million people to the crisis in Italy, but still failed to build testing capacity early enough.

He criticized slow reversals on masking and said the Centers for Disease Control and Prevention lost precious time when its first test design faltered and other laboratories were not allowed to move faster. He also called testing a public good because one person’s test can reduce everyone else’s exposure. In a National Bureau of Economic Research paper by Lawrence Summers and David Cutler, the pandemic’s total cost to the United States was estimated at $16 trillion.

Christakis used one image to show how serious the earliest signal should have been:

“The Chinese passed rules that put 930 million people in home confinement. The Chinese thought that this disease was so bad [...] that they basically detonated a social nuclear weapon.”

What you should take away

• Early testing failures limited the ability to isolate cases before hospitals filled.
• Mask guidance and public communication moved too slowly for a virus that spread before symptoms.
• The economic damage came from the virus itself as well as from attempts to control it.

If you want to hear Christakis unpack testing capacity and the link between public health and the economy, listen to the full episode on Spotify.

Why were vaccines helpful but still not an instant fix?

That backdrop made the vaccine news hopeful, but Christakis warned against treating early efficacy results as an on and off switch. He was impressed by how quickly vaccines were developed, especially after Chinese researchers released the viral sequence and companies such as Moderna moved immediately. Still, manufacturing, distribution, and public uptake were always going to take longer than the headlines suggested.

His clearest caution involved safety interpretation. A trial with about 40,000 people can show strong protection, but it cannot rule out every rare event. Christakis expected the earliest doses to go first to placebo participants and healthcare workers, then wider rollout would build confidence. He returned to that next phase in Episode 144 of the WHOOP Podcast.

His threshold for perspective was numerical:

“The trials have order 40,000 people in them, so rare serious complications [...] 1 in 100,000 people having a serious complication or death, would have been unlikely to have been detected in the trials.”

What you should take away

• Fast vaccine development did not mean instant population level protection.
• Trial size can show efficacy while still leaving very rare risks to post rollout monitoring.
• Early vaccination of high exposure groups helps build real world confidence in safety.

If you want to hear Christakis go deeper on vaccine rollout and rare adverse event logic, listen to the full episode on Spotify.

When does herd immunity happen, and what comes after it?

Even with vaccines moving quickly, Christakis argued that daily life would change only when enough people had immunity to slow transmission across the population. Using an R0 near 3 for SARS-CoV-2, he said the simple herd immunity threshold was around 66%, then noted that network effects could pull the working estimate closer to 40% to 50%. At the time of recording, he thought about 12% of Americans had already been infected.

That framework led to a longer timeline than many people wanted to hear. Christakis expected 2021 and part of 2022 to remain shaped by masks, testing, and physical distancing, followed by a slower social recovery. He also predicted a release phase after the shock passed, with more spending, more socializing, and a return to collective life.

His summary of timing was direct:

“Probably between 40 and 50% [...] of Americans would need to get this disease before we reach the herd immunity threshold.”

What you should take away

• Herd immunity can happen before 100% of people are immune, but it still requires a large share of the population to have protection.
• Christakis expected the transition out of the pandemic to happen in stages rather than all at once.
• Post-pandemic behavior often swings toward more social activity after long periods of restriction.

The bottom line

• SARS-CoV-2 spread widely because infectiousness often began before symptoms.
• Christakis estimated an infection fatality rate of about 0.5% to 0.8% in December 2020, with far higher risk in older adults.
• Testing capacity was a public health tool and an economic tool at the same time.
• Vaccine efficacy did not remove the need for rollout time, safety monitoring, and public uptake.
• Herd immunity depended on population level protection, not on a single scientific milestone.
• WHOOP respiratory rate data gave some people an early signal that illness could be present before symptoms.

Frequently asked questions about things discussed in this episode

How does WHOOP measure respiratory rate?

WHOOP measures respiratory rate during sleep and builds a personal baseline, which makes unusual overnight increases easier to spot.

What does WHOOP do for changes that may point to illness?

WHOOP surfaces changes in metrics such as respiratory rate, resting heart rate, and Heart Rate Variability so you can see when your baseline shifts.

Does WHOOP diagnose COVID-19?

WHOOP does not diagnose COVID-19. WHOOP shows physiological changes that can help you decide when to pay closer attention or seek medical testing.

What does WHOOP show when recovery is under stress?

WHOOP reflects stress through patterns across Recovery, resting heart rate, Heart Rate Variability, Sleep, and respiratory rate rather than through a single illness label.

What can WHOOP members learn from respiratory rate trends?

WHOOP members can learn whether respiratory rate is stable for them over time, which makes a sudden deviation more useful as a personal warning sign.

Tracking respiratory rate beside Recovery, resting heart rate, and Heart Rate Variability gave WHOOP members a practical way to spot the kinds of physiological shifts that mattered throughout this stage of the pandemic.