Agreed but it's still a variable that's unaccounted for but quite important. And herein lies the problem, only if the groups were randomly selected could the data be valid. That's not usually how clinical efficacy trials are done. Pharmaceutical companies have an interest in trying to overstate efficacy. This is done from the outset by selecting individuals that are the most likely to show an effect from the drug and at the same time least likely to show any adverse reactions.
Also what exactly is serious illness? These are mostly young people where serious illness is minimal.
They used the FDA definition of severe Covid:
Of 31 cases of severe, FDA-defined COVID-19, with onset post-dose, 30 occurred in placebo recipients, corresponding to 96.7% VE (95% CI 80.3-99.9) against severe COVID-19
FDA definition from here:
COVID-19: Developing Drugs and Biological Products for Treatment or Prevention
• Positive testing by standard RT-PCR assay or an equivalent test
• Symptoms suggestive of severe systemic illness with COVID-19, which could include any symptom of moderate illness or shortness of breath at rest, or respiratory distress
• Clinical signs indicative of severe systemic illness with COVID-19, such as respiratory rate ≥ 30 per minute, heart rate ≥ 125 per minute, SpO2 ≤ 93% on room air at sea level or PaO2/FiO2 < 300
Also looking at the age group breakdown, there were actually a fair bit of older people. About 40% over 55 and 60% below. Though they were mostly healthy people with few or no comorbidities, hence very few deaths, vaccine or placebo. However 30x more severe Covid-19 in the placebo group.