John P. A. Ioannidis (/ˌiːəˈniːdɪs/ EE-ə-NEE-diss; Greek: Ιωάννης Ιωαννίδης, pronounced [i.oˈanis i.oaˈniðis]; born August 21, 1965) is a Greek-American physician-scientist, writer and Stanford University professor who has made contributions to evidence-based medicine, epidemiology, and clinical research.

Ioannidis's 2005 essay "Why Most Published Research Findings Are False" was the most-accessed article in the history of Public Library of Science (PLOS) as of 2020, with more than three million views.

[1][2] Ioannidis was a prominent opponent of lockdowns during the COVID-19 pandemic, and he has been accused of promoting conspiracy theories about COVID-19 policies and public health and safety measures.

[29] Thomas Trikalinos and Ioannidis coined the term Proteus phenomenon to describe tendency for early studies on a subject to find larger effect than later ones.

[30] He was an early and influential public critic of Theranos, the now-fallen Silicon Valley blood test startup that at its height was valued at up to $9 billion.

[47] In the second paper, he discussed solutions: "adoption of large-scale collaborative research; replication culture; registration; sharing; reproducibility practices; better statistical methods; standardization of definitions and analyses; more appropriate (usually more stringent) statistical thresholds; and improvement in study design standards, peer review, reporting and dissemination of research, and training of the scientific workforce".

[48][49][50] In the third paper, he proposed eight features that are important for useful clinical research: problem base, context placement, information gain, pragmatism, patient-centeredness, value for money, feasibility, and transparency.

[51] Ioannidis was invited to present his findings as a keynote speaker at the "Evidence Live 2016" conference, hosted jointly by the Centre for Evidence-Based Medicine (CEBM) at the Nuffield Department of Primary Care Health Sciences, University of Oxford and the BMJ.

[65] In an essay written to honor his late mentor David Sackett, he stated that "Influential randomized trials are largely done by and for the benefit of the industry.

Risk factor epidemiology has excelled in salami-sliced, data-dredged articles with gift authorship and has become adept to dictating policy from spurious evidence.

He has also advocated for the use of large national population databases with systematically collected data to minimize bias and improve yield of trustworthy discoveries.

[5] He made a rough estimation that the coronavirus could cause 10,000 U.S. deaths if it infected 1% of the U.S. population, but argued that more data was needed to determine how widely the virus would spread.

[128][3][5] The virus in fact eventually became widely disseminated, and would cause more than one million deaths in the U.S.[129][128][3] Ioannidis expressed doubt that vaccines or treatments would be developed and tested in time to affect how the pandemic would unfold.

[131] In March 2020, Ioannidis tried to organize a meeting at the White House where he and colleagues would caution President Donald Trump against "shutting down the country for [a] very long time and jeopardizing so many lives in doing this," according to a proposal he submitted.

The meeting did not come to pass, but on March 28, after Trump said he wanted the country reopened by Easter, Ioannidis wrote to his colleagues, "I think our ideas have inflitrated [sic] the White House regardless".

[3] Ioannidis widely promoted a study of which he had been co-author, "COVID-19 Antibody Seroprevalence in Santa Clara County, California", released as a preprint on April 17, 2020.

It asserted that Santa Clara County's number of infections was between 50 and 85 times higher than the official count, putting the virus's fatality rate as low as 0.1% to 0.2%.

[134] The message found favor with right-wing media outlets, but the paper drew criticism from a number of epidemiologists who said its testing was inaccurate and its methods were sloppy.

[128] Amid controversy over his COVID-19 work and his frequent televised interviews, Ioannidis was harassed in memes and emails, including one falsely claiming his mother died of COVID-19.

[144] In 2022, Ioannidis authored a paper in BMJ Open arguing that signatories of the Great Barrington Declaration were shunned as a fringe minority by those in favor of the John Snow Memorandum.

According to him, the latter used their large numbers of followers on Twitter and other social media and op-eds to shape a scientific groupthink against the former, who had less influence as measured by the Kardashian Index.

"[147] In the same exchange of comments on The BMJ, Ioannidis addressed the concerns of Yamey, Gorski and Meyerovitz-Katz in his "Fourth set of replies", additionally stating that his "COVID-19 papers have been cited about 5 thousand times in the scientific literature by tens of thousands of scientists and were discussed by millions of people," and dismissed conflict of interest by asserting that he did not sign the Great Barrington Declaration or any other petition or signature collection on COVID-19, as he is against the notion that scientific matters and evidence could be decided by signature collections and prefers these matters be handled by heavily moderated public debates.

[155] In 2016, Quartz ran a feature on Ioannidis titled "The man who made scientists question themselves has just exposed huge flaws in evidence used to give drug prescriptions".