Is their another way to increase you immunity in the doomsday event. The only one I know of is upgrade..

Immunity is only increase by upgrading the virus research facility. But if you have earned some speciality points from leveling up (using the honor awarded) then you can use these to let you build more research facilities. Each facility building will give you more immunity and can be upgraded individually.

In the doomsday conquest event menu go to the Speciality section to check your level. Add any honor you have earned and then use any speciality points you have. In the Blue section you need to unlock the individual bonuses moving towards the larger circles diagonally away from the center. They have the bonus of 'All honor structures capacity +1'. For each one of these that you reach you can build an additional one of each honor building, eg the virus research facility.