evidence: Covid-19 vaccines: your questions answered [...] The study also found that people aged 75 and above who did not receive all the doses they were eligible for were twice as likely to become severely ill with Covid-19, compared to those who were fully vaccinated. [...] A study of nearly 46 million adults in England published in July 2024 in the journal Nature Communications found that in the 13 to 24 weeks after the first dose of any Covid-19 vaccine doctors saw 10% fewer cases of blood clots in the arteries (arterial thromboses) including heart attacks and strokes, than they did in those who did not get vaccinated.