At least not when you eliminate the three most common biases present in long COVID research. Most studies of long COVID by vaxx status are subject to recall bias and sampling bias because they rely on self-reported symptoms collected via surveys. While a systematic review found that 7 out of the 9 studies reviewed support the proposition that prior vaxx is associated with a lower incidence of long COVID after infection, all 7 studies relied on self-reported symptoms via a symptom survey. Two of them did not. In their matched cohort study, Taquet and colleagues (n = 18,958) confined their data to attended and diagnosed symptoms in electronic health records of pre-omicron infections during a 6 month observation window and only included unvaxxed patients who had received any prior flu shot which eliminated the recall bias, sampling bias and healthy vaccinee bias present in most long COVID research. After eliminating these three common biases they found that covid vaxx prior to infection was not protective against several documented long COVID symptoms.
‘On the other hand, previous vaccination did not appear to be protective against several previously documented outcomes of COVID-19 such as long-COVID features, arrhythmia, joint pain, type 2 diabetes, liver disease, sleep disorders, and mood and anxiety disorders.’
A longitudinal cohort study conducted in India that followed hospitalized COVID19 patients (n = 487) for a median 223 days found that ‘participants who took two doses of COVID-19 vaccination had higher odds of developing Long COVID’ and tried to attribute the outcome to the collider bias. The largest cohort study of long COVID risk on record (n = 13 million), which is unfortunately behind a paywall, found that vaxx prior to infection only reduced the risk of long COVID symptoms by a measly 15%. You can still read the write up for it by accessing the web archive version of the nature article from 2022. A smaller but similar study (n =64,571) conducted among VA patients with breakthrough infections prior to omicron (observation period ended in August 2021), found that vaxx prior to infection did not result in a statistically significant risk reduction of long COVID symptoms in kidney, gastrointestinal, mental health, and neurologic organ systems. Overall relative risk reduction across all long COVID symptoms was only found to be 13%.
Comparatively, the risk of post-acute sequelae in the cardiovascular, coagulation, metabolic, and pulmonary organ systems, as well as risk of fatigue, was lower in people with breakthrough COVID-19 vs those with COVID-19 without prior vaccination. Lower risk was not statistically significant in kidney, gastrointestinal, mental health, and neurologic organ systems. None of the outcomes exhibited a higher risk among people with breakthrough COVID-19 compared to those with COVID-19 without prior vaccination.
More recently, a Mayo Clinic retrospective cohort study (n = 41,652) that assessed only attended and professionally diagnosed long COVID symptoms, which eliminates recall and sampling bias, found 'no difference in medically-attended and diagnosed post acute sequelae from COVID19 (PASC)between unvaxxed and vaxxed patients who received the full primary series or booster. ' The development of PASC was associated with increasing age, female sex, hospitalization for the initial infection, and an increased severity of comorbidities. Infection during the omicron period (the vast majority of COVID19 cases) was inversely associated with the diagnosis of PASC. Anti-N seroprevalence was much higher at the end of this study's observation period (December 2022) than it is for previous observational research finding a link between vaxx status and long COVID that limited their electronic health record and/or survey data to the much smaller number of infections that occurred prior to omicron. The CDC estimates that >70% of Americans had contracted the virus by the third quarter of 2022 with infection rates >80% for adolescents and young adults between 16-29 years of age. Thus, this study better represents current long COVID symptom risks by extending their observation period a year into the omicron era.
The cluster of symptoms called long COVID are also so poorly defined than they could be the result of non-COVID related illnesses or disorders. A recent emergency department prospective cohort study spanning per-omicron and omicron variants (n = 6,723) found that 1 in 5 test negative emergency department patients (i.e. no confirmed SARS-2 infection at admission) suffered at least one long COVID symptom providing evidence that the condition is over-diagnosed and the defined symptoms are too ambiguous to accurately pinpoint the prevalence of prolonged COVID19 disease. They also found that ‘vaccination was not associated with less PCC (Post Covid19 Condition Symptoms i.e. Long COVID) in patients with or without SARS CoV-2.'