The vaccines are generally safe. Fever occurs in five to tenpercent of children vaccinated. Temporary muscle pains or feelings of tiredness may occur as well. In certain years, the vaccine has been linked to an increase in Guillain–Barré syndrome among older people at a rate of about one case per million doses. Although most influenza vaccines are produced using eggs, they are still recommended for people who have severe egg allergies. However, influenza vaccines are not recommended in those who have had a severe allergy to previous versions of the vaccine itself. The vaccine comes in "inactive" and "weakened" viral forms. The live, weakened vaccine is generally not recommended in pregnant women, children less than two years old, adults older than 50, or people with a weakened immune system. Depending on the type they can be injected into a muscle, sprayed into the nose, or injected into the middle layer of the skin (intradermal). The intradermal vaccine was not available during the 2018–2019 and 2019–2020 influenza seasons.