URGENT: What are the statistical limits and biases in this scientific article?

I need to deliver this exercise in the next 10 hours.

QUESTION: What are the statistical limits and biases in this scientific article, which challenge its validity?

THE ARTICLE: Efficacy of hepatitis A vaccine in prevention of secondary hepatitis A infection: a randomised trial

Abstract =

BACKGROUND: Hepatitis A vaccination stops outbreaks of hepatitis A infection, but its efficacy against infection after exposure has not been proven. We investigated the use of hepatitis A vaccine to prevent secondary infections with hepatitis A virus (HAV).

METHODS: We did a randomised controlled trial of hepatitis A vaccine in household contacts of people with sporadic HAV infection (index cases). Households (index cases and contacts) were randomly assigned to the vaccine group or unvaccinated group, according to the study week in which they were enrolled. All household contacts in the vaccine group received vaccination at the time of entry to the study.

FINDINGS: During 45 days of follow-up, secondary infection had occurred in ten (13.3%) of 75 households (two families had two cases each) in the untreated group and in two (2.8%) of 71 households in the vaccine group. The protective efficacy of the vaccine was 79% (95% CI 7-95). The number of secondary infections among household contacts was 12 (5.8%) of 207 in the unvaccinated group and two (1.0%) of 197 in the vaccinated group. Therefore, 18 individuals needed to be vaccinated to prevent one secondary infection.

INTERPRETATION: Hepatitis A vaccine is effective in the prevention of secondary infection of HAV and should be recommended for household contacts of primary cases of HAV infection.