Pregnant women who contract the H1N1 infection are at a greater risk of developing complications because of hormonal changes, physical changes and changes to their immune system to accommodate the growing fetus. For this reason the Center for Disease Control and Prevention recommends that those who are pregnant are vaccinated to prevent the influenza virus. The vaccination should not be taken by people who have had a severe allergic reaction to the influenza vaccination. Additionally those who are moderately to severely ill, with or without a fever should wait until they recover before taking the vaccination.