There are now 3 vaccines available in the United States against the virus that causes COVID-19. They are all effective, but there are some important differences between them.

Pfizer vaccine

This is a mRNA vaccine given as 2 doses 21 days apart. An mRNA vaccine teaches our cells how to make a protein that creates an immune response. That immune response produces antibodies and protects us from getting infected if the real virus enters our bodies. The Pfizer vaccine is 95% effective in protecting against COVID-19 after the second dose. Most common side effects are fatigue and headache after both doses. However, it is more common to have these side effects after the second dose. Side effects seem to be milder for people over 55 compared with those aged 16 to 55. There is no clinical data about the variants of the virus and the vaccine, but lab studies suggest it may be less effective against the South African variant. Pfizer is studying a third booster dose of the original vaccine against this variant. They also aim to make a new vaccine that is specific to the variant.

Moderna vaccine

This is also a mRNA vaccine given in 2 doses 28 days apart. The vaccine is 94% effective in preventing COVID-19 after the second dose. Common side effects include fever, chills, headache, and muscle aches and pain. These effects have been reported in 60% of participants after the first dose, and in 80% of participants after the second dose. There is no clinical data about the virus variants. In lab studies, there is not an effect on the antibody response to the U.K. variant. However, the vaccine may be less effective in protecting against the South African strain of the virus. Moderna will test a booster dose against these variants and also a third smaller dose of the original vaccine.

Johnson & Johnson vaccine

This is a different type of vaccine. This vaccine uses another virus which has been changed to deliver a gene that tells cells to make an antigen. This triggers antibodies and immune response. The virus used in the vaccine will not make humans sick because it has been changed. This vaccine is currently given as just one dose, but they are currently researching using 2 doses. This vaccine is also very effective, with 72% rate of protection in the U.S. and 66% globally against moderate-to-severe COVID-19. With just a single dose, this vaccine is 85% effective against severe COVID-19. Common side effects include headache (39%), fatigue (38%), muscle pain (33%), nausea (14%), and fever (9%). This vaccine was less effective against the South African (57% effective) and South American (66% effective) variants.


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