The main findings of the study are presented in the table below. Please note the following:

The row labeled "Overall" presents an analysis of the outcomes based on all study enrollees.

The subsequent rows under each head are the results separately for different sub-groups.

"95% CI" is an abbreviation for "95% Confidence Interval", a concept that will be covered starting in Lecture Set 7, and will be revisited in the next assignment.

"Number of cases" refers to the total number of COVID-19 infections incurred among participants after receiving the second dose of the 2-dose vaccine regimen.

Suppose you plan to perform a two sample z-test (or log rank test) to compare the COVID-19 outcome between the vaccinated and unvaccinated groups. What is the null hypothesis for this test in terms of the incidence rate ratio (IRR)?

a. Ho:IRR0:0

b. Ho:IRR=0:=0

c. Ho:IRR1:1

d. Ho:IRR=1:=1