Researchers conduct a study to evaluate the effectiveness of a newly licensed rotavirus vaccine in a developing country. They vaccinate 8,000 children aged 6-24 months in a rural district over 6 months. The baseline annual incidence of severe rotavirus gastroenteritis in the district was 200 per 10,000 children under 5 years of age. Two years after the vaccination campaign, the district's overall disease surveillance system reports 12 cases of severe rotavirus gastroenteritis among all children under 5 in the district (total population 50,000 children). The incidence has declined to 24 per 10,000. Which of the following most accurately describes the limitation of drawing conclusions about vaccine effectiveness from this data?

1. A)The vaccine is clearly 88% effective because the disease incidence decreased from 200 to 24 per 10,000
2. B)The study cannot determine vaccine effectiveness without a comparison group of unvaccinated children with similar baseline riskGABARITO
3. C)The vaccine must be ineffective because cases still occurred in some vaccinated children
4. D)The decline in incidence is attributable to vaccination alone, as evidenced by the improvement in the entire district
5. E)Vaccine effectiveness cannot be calculated because the absolute number of cases (12) is too small