February 21st, 2021

Why Are COVID-19 Case Numbers Dropping?

We don’t know. That part is easy.

Also easy is that case numbers really are falling — it’s not just reduced testing — and it’s happening pretty much everywhere.

Urban areas and rural. Red states and blue. Places with broad vaccine rollouts and those with hardly any. North and South America, Europe, Africa, and Asia. Even countries with the B.1.1.7 variant.


Source: 91-divoc.com/pages/covid-visualization/, downloaded 21 Feb 2021.

Let’s round up some theories:

1. Seasonality. An attractive hypothesis — coronavirus infections pre-SARS-CoV-2 definitely show a seasonal pattern.

And various viral diseases go through communities synchronized with the seasons, especially when school starts or the weather gets colder. Any pediatrician will tell you that.

Note that the term “seasonality” has always been a bit misleading — it refers to infections peaking within seasons, not throughout them. Think of influenza, how sometimes we have an early, sometimes a late seasonal peak in the winter.

The problem with this seasonality theory is that the seasons are flipped in the southern hemisphere. And didn’t cases surge over the summer in many southern U.S. states?

2. Herd immunity. Nearly 28 million Americans have had a confirmed COVID-19 diagnosis reported to the CDC. This represents only a fraction of the true cases — especially the mild or asymptomatic ones — and the CDC estimates that only 1 in 4.6 infections are reported. That could bring us up to half the US population with some degree of natural immunity to infection.

Even as of mid-January, the CDC put the actual case numbers at over 80 million, and certainly it’s higher than that now. And note that in some regions, the actual case counts might be even higher — 5 to 20 times higher, according to one recent publication.

3. Behavior. We know much better now how this virus is transmitted. Avoiding crowds and indoor spaces with poor ventilation — and wearing masks — reduce the risk. But has our behavior actually followed suit?

The holidays are behind us. The Super Bowl was lousy. Not many parties for the Australian Open tennis finals. Spring Break hasn’t happened yet.

One compelling hypothesis, related to herd immunity, is that the people least likely to follow infection control advice — or unable to follow it based on work or living situation — already have had COVID-19 and hence are immune.

The others, not yet infected, watched cases surge in December and January and continue to hunker down and stay safe — or again, have the luxury of staying safe. They might be especially vigilant now that a vaccine is in their not-too-distant future — you know, the pot of gold at the end of the rainbow, the light at the end of the tunnel, or the Holy Grail at the end of the Monty Python movie.

(Sorry about that.)

4. Vaccines. The world is vaccinating like crazy. Demand is off the charts. And in most places, we’re targeting the people most likely to have symptomatic or severe disease.

Plus, the data increasingly suggest that the vaccines reduce not just disease, but also the likelihood of transmission — they reduce infections overall (uninfected people can’t transmit), and those with infection have lower viral loads. 

While the vaccine rollout is not yet broad enough to explain the case number drop on its own, it might be contributing. It certainly could be playing a role in Israel.

5. The virus. Maybe the virus is doing us a favor and becoming less virulent over time. Perhaps some of these variants — if not B.1.1.7 — in order to gain the ability to transmit, also cause less severe disease.

Take the virus’s perspective — yes, think like a virus — and how this would be evolutionarily beneficial. More mild cases, more chance to spread its genetic material to other susceptible hosts. That’s all viruses care about, right?

6. It’s a gemish. This brings us to the most likely explanation for the drop in cases, a gemish — Yiddish for a mixture of things. (It’s pronounced “ga-mish”, in case you want to try it out on your own.)

It could be all of the above explanations, in various proportions, and different in various regions — plus things no one has considered.

And the uncertainty about why cases are dropping again hearkens back to this great H.L Mencken quotation, which over time has morphed into this profound statement:

Every complex problem has a solution which is simple, direct, plausible — and wrong.

I stress the importance of being humble about not knowing why the cases are dropping simply because reliance on one of these factors over another could get us into trouble. For example, this week Dr. Marty Makary, writing in the Wall Street Journal, posited that we are already close to herd immunity, making this bold prediction:

There is reason to think the country is racing toward an extremely low level of infection. As more people have been infected, most of whom have mild or no symptoms, there are fewer Americans left to be infected. At the current trajectory, I expect Covid will be mostly gone by April, allowing Americans to resume normal life.

Warning — if anyone tells you with confidence that they know precisely why cases are dropping, and that they have an accurate crystal ball showing that by April we’ll be safely out of this pandemic — please view it with the appropriate scientific skepticism it deserves.

Look, we can hope this optimistic prediction is correct — we all want that. April isn’t far away, we’ll know soon.

But if there’s one thing a pandemic from a new human disease teaches us, it’s that there’s a lot we don’t know.

36 Responses to “Why Are COVID-19 Case Numbers Dropping?”

  1. Leslie Kasza says:

    Humble, modest and brilliant op ed. Superb. Thank you, Dr. Sax.

  2. Adriaan de Haan says:

    You forgot about another large factor – more effective treatments, especially early treatments.

    • Roger Kimber says:

      roger.kimber@pennmedicine.upenn.edu & rgrkimber@gmail.com
      Yes, here’s a shout out to the FLCCC Alliance and there I-MASK+ prophylaxis and early treatment protocols, disdained by many but not so by those of us on the front lines, in the trenches all over the world (flccc.net)

    • Arturo Azpiroz says:

      The early use of anti-SARS-CoV2 antibodies in people at high risk of developing into severe COVID-19 reduces the number of possible deaths, but this does not affect the number of cases or incidence of the infection at all.

  3. Birchley Poundbottom says:

    Warning — if anyone tells you with confidence that they know precisely why cases are rising, and that they have an accurate crystal ball showing that in two weeks we’ll be crushed under a exponential surge— please view it with the appropriate scientific skepticism it deserves.

    • Amanda Ryder says:

      Please don’t apologize for the Monty Python reference. Any and all Monty Python references are welcome

  4. Kimon Zachary says:

    Indeed a gemish, but I think mostly #3. And hopefully this won’t end like the Minty Python movie.

  5. Carl G Weber says:

    Great article! Thanks. Seems kinda early to be due to seasonality.
    I personally think it was all due to the double masking + keeping the schools closed :>)

  6. Jacobo TIEFFENBERG says:

    Dear Paul: your article is so accurate. I think the concept of “all of the above” is the most acceptable given the uncertainty we live in. But we must make decisions in the uncertainty:
    Vaccinate as fast as possible; first the sanitary personnel; then the high risk groups (by job, age and conditions) the teachers! so we can safely send teachers to our kids and our kids to school.
    And then mantain all precautionary measures already agreed, social distance and masks, open sites, ventilated places.
    Let the time go by, and accumulate information that will allow for more accurate measures.
    And preprare for the next ID threat. Given the creazyness of our world, we should be better prepared.
    Thank you
    Jacobo Tieffenberg

  7. Eduardo Q Araujo says:

    Sorry to be a wet blanket, but this certainly is NOT the case here in Brazil. We´re facing the worst of the pandemic and witnessing uncountable cases of people developing severe disease even after getting one jab of either Oxford´s or Coronavac vaccine (many of them young without comorbidities). I wouldn´t count the chickens before they hatch (at least not in countries without access to Pfizer´s or Moderna´s vaccine).

    • Arturo Azpiroz says:

      The data that have been arriving about the effectiveness of CoronaVac (a classic type vaccine with culture and inactivation of SARS-CoV2) are quite variable depending on the place of application, for example more than 90% in Turkey to more than 50% in Brazil, however, a 100% preliminary efficacy is published in the prevention of severe COVID-19.

      • Eduardo Q Araujo says:

        The caveat is that the trial was conducted before the rise of those new variants, so there remains doubt regarding its efficacy against them

  8. Pierre Rimbaud says:

    Another idea : the likelihood to be infected varies with individuals ; if the most receptive people have already been infected, th R0 drops.

  9. Ken Pepperdine says:

    Perhaps there’s the “Terrain Factor” ,for despite little or no guidance from health agencies, many individuals have opted to enhance their own immune response with Vitamins C, D, also zinc selenium , quercetin, NAC. and others as the pandemic persists.

  10. Peter Van der Bijl says:

    This history of pandemics shows three waves each time in different seasons.

    Dynamics of pandemics
    Spanish flu pandemic is reported to spread in three waves: first wave in spring 1918 was moderate but rapidly spread, second wave in autumn 1918 was completely severe and destructive and third wave in spring 1919 was more severe than the first wave but not worse than second wave2. Many countries experienced the second and third waves of more virulent form of infection. It is estimated that 50% of the world’s population have infected, 25% manifested clinical signs [4]. After almost forty years of Spanish flu, a new influenza strain was detected in China. The 1957 pandemic was caused by A H2N2 strain [5]. The virus spread to Hong Kong, Singapore, Taiwan, and Japan and in summer spread to world in the summer of 1957. Two main routes of spread were: firstly, across Russia to Scandinavia and Eastern Europe, and secondly from the US to other countries. Asian flu pandemic had spread to the world in 6 months. The pandemic affected approximately 40 ± 50% of people, and 25 ± 30% of them had clinical signs. Most of the deaths were due to secondary bacterial pneumonia. Mortality rate was calculated as 1 per 4000. The mortality was high among children and elderly people [1]. The 1957 (Asian) influenza pandemic, estimated the basic reproduction number (R0) was 1.8 and 60%–65% of infected individuals were manifested clinical symptoms [6].

    Excess mortality due to Spanish flu was 598 deaths per 100,000 people per year. However excess mortality of Asian flu was only 40.6% [7].

    After a decade occurrence of Asian flu pandemic, the new flu strain H3N2 was caused a new pandemic known as the Hong Kong flu in 1968. Although new virus was extremely transmissible, degree of severity was milder than the Asian flu. Excess mortality of Hong Kong flu was 16.9%. Virus was spread through Vietnam War veterans returning to the United States. Then Hong Kong flu was seen in Japan, England, Wales, Australia, and Canada in 1969 [8,9]. At this pandemic the characteristic mortality shift towards younger populations, the case fatality rate was highest among children [10]. Hong Kong flu is estimated to have caused between 500,000 and two million deaths worldwide in two waves [11]. The burden of pandemic was higher in countries with increases in excess all-cause mortality of 9.1%–13.0%, than in US. Impact of pandemic was present the geographic heterogeneity [8]. Hospitalization was significantly high amongthe elderly. Hong Kong flu pandemic showed that public health intervention strategies and medical science were insufficient and not improved between the 1957 and 1968 pandemics [12].

    In April 2009, a new pandemic occurred that H1N1pdm09 virus was causative agent, emerged from Mexico. Within few weeks, the disease had spread across many countries. The global trade and travel served swine flu to spread as 122 countries in six weeks, however past pandemics had spread in six months [13]. H1N1 pandemic profile had a course with three waves in spring, summer, and fall. The pattern was generally mild wave in the spring and early summer, but reemerged more severe after opening the schools. Studies showed that adults older than 50 years of age were less susceptible to 2009 H1N1 infection than younger adults. While compared the previous pandemics, the infectivity was higher among children than among adults (Table) [14]

  11. S. Carrier MD FACEP says:

    If you’re going to tell me that its NOT due to decreased testing then show me the numbers. I do not think that the amount of testing going on is the same as it has been. People are spending their time and energy on figuring out how to get the vaccine, not on getting tested.

    • Arturo Azpiroz says:

      I remember that a few months ago I read the news that the director of the CDC established the indication to perform RT-PCR for SARS-CoV2 only for symptomatic individuals who came to medical centers, that is, as a diagnostic means. The next day he retracted due to a few hundred opinions to the contrary.
      Today the hodgepodge of “PCR” tests is such that no one really knows what a positive or a negative means in asymptomatic people.
      I hope the decrease in the performance of these useless tests in the way of herd immunity is true.

  12. Jose Perez-Rodriguez says:

    To Peter Van der Bijl: Where can one get the references that you mention in your comment? Thanks.

  13. Sanford R. Kimmel says:

    I have great respect for Dr. Sax but would take issue with his comment from # 4 that “in most places, we’re targeting the people most likely to have symptomatic or severe disease. Every news report that I have seen indicates that in the United States, the wealthy have much higher vaccination rates than those from poorer socioeconomic groups e.g. Greater than 50% on Fisher Island, Florida vs.3% in a nearby zip code. Even in Israel, the vaccination rate for Palestinians is less than other Israeli’s (By the way, I’m Jewish). We have a long way to go to achieve vaccine equity.

    • Jason Greenworth says:

      Thanks Sanford. So are saying that vaccines are not responsible for the decline in numbers because there isn’t vaccine equity? The blog was about reduction in infection rates, not one’s impression of fairness in vaccine delivery.

  14. William Schuler says:

    The BBC would like to apologize for the preceding sketch, skit, and spoof…

  15. Jim Recht, MD says:

    Dr. Sax’s reference to Israel ignores the fact that (according to Amnesty International “while it celebrates a record-setting vaccination drive, millions of Palestinians living under Israeli control in the West Bank and the Gaza Strip will receive no vaccine or have to wait much longer.” According to the World Health Organization, as of 2/17/2021 there have been 192,791 cases of COVID-19 in the OPT and 2,147 Palestinians deaths from COVID-19. Health professionals are urged to read this week’s statement from Jewish Voice for Peace’s Health Advisory Council (https://mondoweiss.net/2021/02/israel-must-provide-covid-vaccines-to-palestinians-in-the-west-bank-and-gaza/?amp=1). Signatories include our colleagues Alan Meyers, MD, MPH; Alice Rothchild, MD; Amy Alpert, CCC-SLP; Maxine Fookson, RN, MN; Peter Sporn, MD; Rachel Rubin, MD, MPH; S. Komarovsky, MPH: Sima Kahn, MD; and
    Trude Bennett, MSW, DrPH.

  16. Jim Recht, MD says:

    Replying to Jason Greenworth: yes, the issue is in fact equity. Equity in this case would include distribution of the vaccine to the communities most at risk. In the U.S. those are poor communities of color. In Israel those are poor communities in the occupied territories. Equity in this case would mean more effective use of the vaccine — at least potentially, more effective control of COVID-19 spread.

  17. Andrew P says:

    Maybe interim “herd immunity” has been achieved for the current circulating variants, at least for people willing to go out and do things where they might be infected, and that will only continue until new variants emerge that escape existing immunity, or the people currently shutting themselves in decide to go out.and socialize.

    If some right wing young women want to reinvigorate the pandemic and destroy Joe Biden’s Presidency, they could date huge numbers of men with a variety of different variants, so their own bodies would become Petri dishes and cook up new variants by recombination. I say young women because their own personal risk from coronaviruses is minimal.

    • Arturo Azpiroz says:

      I do not know if what I have understood is correct or not.
      I prefer to think that your Google translator program or mine (or both) are somewhat damaged (Is it due to a virus?)

  18. Dr. Denise Kramer says:

    Theory 1: Have you thought about the influence of climate changes? Not only temperature and UV-radiation strength, but also relative humidity outside? There sure will be an influence of sedimenation speed of aerosols, UV-decontamination etc.

  19. Timothy McAloon says:

    Wouldn’t a reason that the southwestern United States experienced an increase in virus cases last summer was due to being indoors more frequently? Unlike the majority of the US who venture outdoor in the summer months, the opposite occurs in states with severe heat. Those residents seek the comfort of indoor air conditioning.

  20. Mike Page says:

    Much respect for this editorial BUT missed two of the biggest likely contributory factors:

    1. Hasn’t the widespread PCR testing been modified in some areas – reduction of ‘amplification cycles’ greatly reduces false positives.

    2. Increased reluctance to test because of having to self quarantine if positive -a big disincentive to many in work etc who may not be at major risk from the infection.

    Other wise vaccination uptake will be helping although this can be offset by new variants at any stage unless we can get sufficient her immunity to decrease this potential.

  21. Ethan Bindelglas says:

    To Dr. Recht,
    Unfortunately, your misguided comments fail to underscore the efforts Israel has made to vaccinate all it’s citizens whether Muslim, Christian, or Jewish in an effort to reduce Covid transmission. Indeed, they have shown a decrease in Covid infection among those vaccinated, which was the point Dr. Sax was making. Why would you turn a blog about the complexity of decreasing infection rates into a politicized discussion? There are plenty of other forums for that.

  22. Todd Carlson says:

    Great summary, especially because many of the lay public tend to oversimplify things, especially in regard to their self–diagnosis and self-treatment.

    Also, I love that you were able to slip “gemish” in! My kids have grown up knowing that word, (although neither of us speak Yiddish.) It’s almost the same in German – Gemisch.

  23. Thomas Kaspar says:

    I think the reason so many seem to be “uninfectable” in spite of high exposure is the T cell immunity that is always present in great vaccines and apparently some of us havew T cells that recognize something COVID shares with previous and similar Coronaviral exposure.
    Say all you want TONY, these antibody tests ( B cell driven ) are not very predictive because even long after antibodies disappear, those T cells remember.
    reason #5 also is probably important too since viruses tend to naturaly become more contagious and less deadly because those are huge reproductive advantages.
    Behavior may have had a big impact on influenza but COVID still hit every single county in the US and California lockdowns did no better and maybe worse than wide open Florida. I think the lockdown just prolonged and possibly worsened the inevitable spread to the entire US.

  24. Andrew M Norris says:

    You forgot to include the main reason:
    President Trump is now out of office and Biden is president.
    That is the REAL reason.

  25. D.L. Stanton, MD says:

    Brazil got hammered awhile back and now recently has turned around. Manaus, a very large city deep in the Amazon documented 76% of population with immunity from vaccination and T Cell immunity from previous exposure , natural immunity. This was reported in the Lancet by British docs. Sure sounds like herd immunity is getting close and kicking in, altho lots more vaccinations are planned.

    Also here at home, N.Dakota just had 50 case and NO deaths reported by Dr Makary at Johns Hopkins. Makary says they have reached herd immunity between vaccinations and TCell immunity, and i believe him . Furthermore he believes we will now start seeing regions in US with #cases falling off precipitously, as well as deaths dropping way down. The sooner we get more folks vaccinated the better. Add in T Cell immunity and we are in fact, very close to herd immunity in some areas. Makary’s point about local areas improving from here on out sure sounds like the answer, but no question much improved Rx has helped immensely with covid patients, as well as finally people got some religion and started wearing masks and distancing more. As was said a gemish….but mainly because of so much natural immunity that has taken hold. By virtue of this places like NYC and LA metropolis likely are going to be last to come around. And yes we still should be wearing masks, not so much for our sakes but for the sakes of the non believers…and also those not yet vaccinated or those who do not have T cell immunity

  26. Julian Ciaha says:

    Why are case numbers dropping worldwide? I am of the understanding that in mid January 2021 the WHO quietly instructed labs to lower the Cycle Threshold value (Ct) that it has previously recommended to use in early 2020 which was absurdly high at 42 to 45…and by doing so the percentage of high false positives from the PCR testing for covid dropped significantly and with it so did the covid case numbers. Some might say conveniently timed for the covid vaccine rollouts and so making the vaccines get the credit for the dropping case numbers. It certainly would not surprise me that the WHO and Pharma would collude to do such a thing after the 2009 swine flu scam-demic(as the Council of Europe called it) the WHO and Big Pharma inflicted upon the world.

HIV Information: Author Paul Sax, M.D.

Paul E. Sax, MD

Contributing Editor

NEJM Journal Watch
Infectious Diseases

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