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Havana syndrome (formally "anomalous health incidents" ) is a disputed medical condition reported primarily by U.S. intelligence and military officials stationed in overseas locations. Reported symptoms range in severity from pain and ringing in the ears to cognitive dysfunction and were first reported by U.S. and Canadian embassy staff in Havana, Cuba.

In February 2022, the State Department released a report by the JASON Advisory Group, which stated that it was unlikely that a directed energy attack had caused the health incidents.

According to David Relman, these findings do not exclude that a weapon could have injured the government workers, as "the most sophisticated brain scans can miss a subtle brain injury, especially if the brain has had time to heal".>

Psychogenic causes
Some scientists have suggested that most cases of Havana syndrome are psychogenic in nature, and are a form of somatic symptom disorder, conversion disorder, or mass psychogenic illness. They state that a psychogenic cause is the only explanation of Havana syndrome that is consistent with the vast majority of cases. In particular, they point to lack of evidence of attacks by hostile nations, and lack of medical evidence of damage to brain or health of purported victims.

Some scientists have suggested that some cases of Havana syndrome were the result of the stress of working overseas in countries that were former cold-war adversaries, where the job often involved anxiety of possible surveillance by the hostile nation. Some scientists have suggested that some cases of Havana syndrome are post-traumatic stress disorder (PTSD) caused by such stress.

Professional reluctance
Some commentators have suggested that the psychogenic hypothesis was downplayed by scientists studying Havana syndrome, for fear of offending the scientists that were supporting the "attack by hostile powers" hypothesis. Specifically, some commentators suggested that some medical professionals were afraid of retaliation if they criticized the conclusions of scientists that were more senior or in positions of power.

Postulated hostile adversaries
In 2018 and 2019, following the first reports of Havana syndrome, the U.S. government postulated that the cause was hostile adversaries attacking their embassy staff, as well as Canadian embassy staff, with sonic weaponry. Later, some scientists disputed that sound waves could cause some of the symptoms being reported, and investigators speculated the recordings may have been of malfunctioning Cuban surveillance equipment. When it was determined that the recorded sounds of supposed attacks were the mating calls of a variety of indigenous crickets, a call was made for "more rigorous research into the source of the reported symptoms." Subsequent studies speculated that directed electromagnetic weapons (using microwaves or radio waves) were the cause. In 2021, James Lin of the University of Illinois claimed that "many researchers and government people have come to believe that the microwave auditory effect — induced by a targeted beam of high peak-power pulsed microwave radiation—may be the most likely scientific explanation for the Havana Syndrome". However, studies published in 2023 and 2024 did not find any evidence of hostile attacks and cast doubt on the idea that electromagnetic energy could produce symptoms consistent with symptoms of Havana syndrome.

Events
The original 21 events in Cuba were characterized as starting with strange grating noises coming from a specific direction. Some people experienced pressure, vibration, or a sensation comparable to driving a car with the window partly rolled down. These noises lasted from 20 seconds to 30 minutes and happened while the diplomats were either at home or in hotel rooms. Other people nearby (including family members and guests in neighboring rooms) did not experience the same symptoms.

2017
In 2017, Robert E. Bartholomew proposed that the most plausible explanation for the newly announced illness reports associated (which would later come to be called Havana syndrome) was a mass psychogenic illness, a condition characterized by the rapid spread of symptoms within a cohesive social group without an organic cause. Bartholomew noted that outbreaks of this condition are often incubated in atmospheres of anxiety.

2018
In March 2018, Kevin Fu and a team of computer scientists at the University of Michigan reported in a study that ultrasound—specifically, intermodulation distortion from multiple inaudible ultrasonic signals—from malfunctioning or improperly placed Cuban surveillance equipment could have been the origin of the reported sounds.

2018 JAMA report
At the U.S. government's request, University of Pennsylvania researchers examined 21 affected diplomats posted to Cuba, and the preliminary results were published in the Journal of the American Medical Association (JAMA) in March 2018. The researchers found "no evidence of white matter tract abnormalities" in affected diplomats beyond what might be seen in a control group of the same age, but described "a new syndrome in the diplomats that resembles persistent concussion". While some of those affected recovered swiftly, others had symptoms for months. The study concluded that "the diplomats appear to have sustained injury to widespread brain networks."

In 2018, Douglas H. Smith, the lead author of a University of Pennsylvania study of 21 affected diplomats in Havana published in JAMA, said in an interview that microwaves were "considered a main suspect" underlying the phenomenon. A 2018 study published in the journal Neural Computation by Beatrice Alexandra Golomb rejected the idea that a sonic attack was the source of the symptoms and concluded that the facts were consistent with pulsed radiofrequency /microwave radiation (RF/MW) exposure. Golomb wrote that (1) the nature of the noises the diplomats reported was consistent with sounds caused by pulsed RF/MW via the Frey effect; (2) the signs and symptoms the diplomats reported matched symptoms from RF/MW exposure (problems with sleep, cognition, vision, balance, speech; headaches; sensations of pressure or vibration; nosebleeds; brain injury and brain swelling); (3) "oxidative stress provides a documented mechanism of RF/MW injury compatible with reported signs and symptoms"; and (4) in the past, the U.S. embassy in Moscow was subject to a microwave beam called the Moscow Signal.

A March 2018 editorial in JAMA by two neurologists argued that a functional disorder such as persistent postural-perceptual dizziness ("a syndrome characterized primarily by chronic symptoms of dizziness and perceived unsteadiness, often triggered by acute or chronic vestibular disease, neurological or medical illness or psychological distress") could explain some of the symptoms the diplomats in Cuba experienced.

The editorial board of the journal Cortex published an editorial referring to the JAMA research's "gross methodological flaws" and registering concern that it had been published. In the board's view, "Allowing such confused and conflicting explanations of methodology and analysis to pass unchallenged is a slippery path for science, and dangerous for society at large".

2018 JASON report
In 2018, JASON, a group of physicists and scientists who advise the U.S. government, analyzed audio recordings from eight of the original 21 incidents of Havana syndrome and two cellphone videos taken by one patient from Cuba. It concluded that the sounds in the eight recordings were "most likely" caused by insects and that it was "highly unlikely" that microwaves or ultrasound beams were involved, because "No plausible single source of energy (neither radio/microwaves nor sonic) can produce both the recorded audio/video signals and the reported medical effects." The group determined with "high confidence" that the two videos were sounds from the Indies short-tailed cricket, and also noted a "low confidence" hypothesis that the noises may have been from a concrete vibrating machine with worn bearings. The report's findings were first reported by Reuters in July 2019.

2019
In January 2019, biologists Alexander L. Stubbs of the University of California, Berkeley and Fernando Montealegre-Z of the University of Lincoln analyzed audio recordings made in Cuba, which were made by U.S. personnel in Havana during incidents associated with Havana syndrome. The conclusion was that the sounds were the calling song of the Indies short-tailed cricket (Anurogryllus celerinictus) rather than a technological device. Stubbs and Montealegre-Z matched the song's "pulse repetition rate, power spectrum, pulse rate stability, and oscillations per pulse" to the recording. Stubbs and Montealegre wrote, "the causes of the health problems reported by embassy personnel are beyond the scope of this paper" and called for "more rigorous research into the source of these ailments, including the potential psychogenic effects, as well as possible physiological explanations unrelated to sonic attacks." This conclusion was comparable to a 2017 hypothesis from Cuban scientists that the sound on the same recording is from Jamaican field crickets.

In response to a December 2017 State Department request, the U.S. Centers for Disease Control and Prevention conducted a "Cuba Unexplained Events Investigation". The two-year investigation of the medical records of 95 U.S. diplomats and family members in Havana who reported symptoms resulted in a final report, marked for official use only, dated December 2019.

In a 2019 paper, Robert Bartholomew and Robert Baloh propose that the syndrome represents mass psychogenic illness rather than a "novel clinical entity". They cite the vagueness and inconsistency of symptoms as well as the circumstances they developed in (affected staff would have been under significant stress as the U.S. had just reopened its embassy in Cuba) as a cause.

2019 JAMA report
Ragini Verma, the lead author of a University of Pennsylvania study published in JAMA in 2019 that found brain differences in diplomats, concluded that based on its findings, "a wholly psychogenic or psychosomatic cause was very unlikely". Verma's results were criticized by several scientists, including one that noted that 12 of the 21 ostensible victims had a history of concussions prior to traveling to Cuba.

2020
In 2020, a book by Bartholomew and Baloh, Havana Syndrome: Mass Psychogenic Illness and the Real Story Behind the Embassy Mystery and Hysteria, was published; it argued in support of the psychogenic illness hypothesis.

NASEM report
In December 2020, a study by an expert committee of the U.S. National Academies of Sciences, Engineering, and Medicine (NASEM), commissioned by the State Department, released its report, concluding, "Overall, directed pulsed RF energy ... appears to be the most plausible mechanism in explaining these cases among those that the committee considered", but that "each possible cause remains speculative" and that "the report should not be viewed as conclusive".
 * Chaired by David Relman, the committee included Linda Birnbaum, Ronald Brookmeyer, Caroline Buckee, Joseph Fins, David A. Whelan, and others. The panel said that lack of information and direct evidence (such as medical testing data about affected persons) limited what it could conclude about the phenomenon.
 * Chaired by David Relman, the committee included Linda Birnbaum, Ronald Brookmeyer, Caroline Buckee, Joseph Fins, David A. Whelan, and others. The panel said that lack of information and direct evidence (such as medical testing data about affected persons) limited what it could conclude about the phenomenon.
 * Chaired by David Relman, the committee included Linda Birnbaum, Ronald Brookmeyer, Caroline Buckee, Joseph Fins, David A. Whelan, and others. The panel said that lack of information and direct evidence (such as medical testing data about affected persons) limited what it could conclude about the phenomenon.
 * Chaired by David Relman, the committee included Linda Birnbaum, Ronald Brookmeyer, Caroline Buckee, Joseph Fins, David A. Whelan, and others. The panel said that lack of information and direct evidence (such as medical testing data about affected persons) limited what it could conclude about the phenomenon.

The 2020 NASEM study found that it was unlikely that "acute high-level exposure to OPs and/or pyrethroids contributed" to the illnesses, due to a lack of evidence of exposures to those pesticides or clinical histories consistent with such exposure, but the study committee "could not rule out the possibility, although slight, that exposure to insecticides, particularly OPs, increased susceptibility to the triggering factor(s) that caused the Embassy personnel cases". NASEM also found it "highly unlikely" that an infectious disease (such as Zika virus, which was an epidemic in Cuba in 2016–17) caused the illnesses.

The 2020 NASEM analysis reads, "the likelihood of mass psychogenic illness as an explanation for patients' symptoms had to be established from sufficient evidence" and "could not be inferred merely by the absence of other causal mechanisms or the lack of definitive structural injuries". In its assessment of potential social and psychological causes, the committee notes the possibility of stress-based psychological responses, and that these were more likely to be triggered by potential threats attributed to human sources than other stressors. It concludes that these could not have caused the acute "audio-vestibular" symptoms some patients experienced, such as sudden unexplained sounds. The scope of the provided data limited the committee's ability to investigate psychological and social factors.

2021
In January 2021, George Washington University's National Security Archive obtained the 2018 CDC report, pursuant to Freedom of Information Act requests. (Some material in the released report was redacted for medical privacy reasons.) The CDC developed a "case definition" of Havana syndrome, consisting of a biphasic (two-stage) syndrome. The first phase of symptoms (sometimes closely after an auditory or sensory event) consisted of one or more of the following symptoms: head pressure, disorientation, nausea or headache, vestibular disturbance, or auditory or visual syndromes. The second phase of symptoms, occurring sometime later, consisted of cognitive deficits, vestibular disturbances, or both. The report concluded, "Of the 95 persons whose medical records CDC evaluated, 15 had illnesses that met the criteria for a presumptive case definition. CDC classified 31 others as possible cases and the remaining 49 as not likely to be a case." The CDC concluded, "The evaluations conducted thus far have not identified a mechanism of injury, process of exposure, effective treatment, or mitigating factor for the unexplained cluster of symptoms experienced by those stationed in Havana."

March 2023 U.S. intelligence agencies' report
On March 1, 2023, the House Intelligence Committee released a report, titled "Intelligence Community Assessment", which was jointly prepared by seven U.S. intelligence agencies. The report concluded "that there is no credible evidence that a foreign adversary has a weapon or collection device that is causing AHIs". The agencies preparing the report reviewed thousands of possible cases of Havana syndrome. The reported stated that there continues to be scientific debate about whether a weapon could produce such health effects.

Five of the seven agencies involved in generating the report concluded "the available intelligence consistently points against the involvement of US adversaries in causing the reported incidents" and that a foreign adversary's involvement was "very unlikely". One of the other agencies concluded that foreign involvement was "unlikely", and the seventh agency declined to make a finding.*

Two of the seven agencies had "high confidence in this judgment while three agencies have moderate confidence". Two other agencies judged "that deliberate causal mechanisms are unlikely to have caused AHIs" but those agencies had "low confidence because they judge(d) that radiofrequency (RF) energy is a plausible cause for AHIs, based in part on the findings of the IC Expert Panel and the results of research by some US laboratories."