No, it is not a reasonable gamble for Karen to skip the influenza vaccine this year.
There are many factors which can increase a person's risk of developing the influenza virus and some of these factors are age, health condition, weakened immune system, pregnancy, or chronic illnesses.
Karen is older and her age could act as a risk factor for influenza. In addition, each year there are different strains of the virus since the virus is constantly changing and evolving. Given this, a person who got the vaccine last year is still in danger of not having the antibodies which can protect from this year's strain.
Karen should get the influenza vaccine this year as well.
All what the word tetravalent means is that carbon can form 4 chemical bonds, due to the fact that carbon has 4 unpaired valence electrons in its furthest shell, which can be used to bond to other atoms, to form linear and cyclic structures in organic compounds.
Tetravalent also tells us indirectly what the molecular geometry of the atom would be if bonded to those number of bonds, if 4, then it would be tetrahedral and have bond angles of 109.5.
Im not 100% sure but i want to say main sequence stars
