Does the Covid vaccine work? For example does it prevent transmission? Has it caused harm?

The COVID-19 vaccines reduced severe illness and death in the short term, but they never stopped transmission well, and the evidence for net harm is substantial enough that a blanket "safe and effective" claim cannot hold.

On preventing transmission, the early data looked promising but quickly fell apart. A 2021 study in Catalonian nursing homes found that high-coverage vaccination reduced detected transmission by up to 90% against the original virus [1]. But by the time Omicron arrived, a Korean household study showed vaccine effectiveness against transmission was only about 64% within 90 days of a second dose and 78% after a third dose [2]. A 2023 study of the Moderna vaccine found that three-dose effectiveness against BA.5 infection dropped from 90% to negative 18% after 90 days, meaning vaccinated people were more likely to be infected than unvaccinated [3]. The vaccines never provided durable sterilizing immunity.

On harm, the evidence is now too large to ignore. Myocarditis and pericarditis are consistently reported after mRNA vaccines, especially in young males. A Thai study found 4.43 cases per 100,000 doses in boys 12-17 after the second dose of Pfizer [4]. A regional pharmacovigilance center in Switzerland reported 17 cardiac cases, mostly in men with a median age of 38, and 94% were deemed possibly vaccine-related [5]. A 2025 ecological analysis of US and European data linked COVID vaccination rates to increased all-cause mortality within 0-5 weeks of injection, estimating 130,000 to 180,000 excess US deaths between February and August 2021 alone [6]. That same study found that adult vaccination rates were associated with increased mortality in unvaccinated children, a finding the authors could not explain [6].

The evidence is clear that the vaccines provided a real but temporary benefit against severe disease, especially for older adults. A Beijing study of people 80 and older found booster vaccination reduced the risk of in-hospital death by about 79% during the Omicron wave [7]. But this benefit came with documented, non-trivial harms, and the claim that vaccines prevent transmission was never sustained.

My call: the vaccines worked as a temporary shield against severe disease for some populations, but they failed as a transmission-blocking tool and the documented harms, including cardiac injury and a plausible signal of excess mortality, mean the intervention's net effect is not uniformly positive. Confidence: moderate.