How effective is the first shot of the Covid-19 vaccine?

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As Covid-19 vaccinations slowly become available to more people, questions remain about the two-dose format. Do you need the second dose at all? And how safe are you after just one?

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Not all vaccines are created equal. Single-dose Covid-19 vaccines do exist — like the Johnson & Johnson vaccine, but you should absolutely get both doses if you receive a two-dose vaccine. And all vaccines take several weeks to kick in.

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Here are the Covid-19 vaccines available in the U.S. now and in line for approval, and what you need to know about them.

Pfizer — 2 doses

The CDC recommends the Pfizer vaccine for use in people aged 16 and older. Its two doses are given 21 days apart.

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Pfizer’s data show a weak immune reaction 12 days after the first dose, which they say is 52 percent effective at preventing infection. After the second dose, the vaccine is 95 percent effective.

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The studies backing the Pfizer vaccine involved participants who received two doses, so there’s no way to tell how effective one dose is long-term. Researchers warn that partial immunity from a single dose could raise the risk of vaccine-resistant strains of Covid-19.

Moderna — 2 doses

The CDC currently recommends the Moderna vaccine for use in people aged 18 and older. Its two doses are given 28 days apart.

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Moderna’s research shows 80.2 percent effectiveness after one dose and 95.6 percent effectiveness after both doses. As with the Pfizer vaccine, the long-term effectiveness of a single dose is unknown.

AstraZeneca — 2 doses

The AstraZeneca vaccine may become available in the U.S. in April, but concerns about the company’s efficacy data could delay roll-out.

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According to the disputed data, the first jab appears to be 76 percent effective in protecting against Covid-19. The second shot appears to be 79 percent effective against symptomatic illness, and 100 percent effective against severe illness and hospitalization.

Johnson & Johnson — 1 dose

The CDC currently recommends the Johnson & Johnson vaccine for use in people aged 18 and older.

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The only single-dose vaccine available in the U.S. now, the Johnson & Johnson vaccine is considered 85 percent effective against severe Covid-19 28 days after receiving the shot.

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According to Dr. Monica Gandhi of UC San Francisco, the vaccine may increase in effectiveness after the first 28 days.

In short:
  • The Johnson & Johnson vaccine is the only single-dose vaccine
  • Every vaccine takes a few weeks after you get it to reach full effectiveness — so you won’t be fully protected immediately
  • The long-term effects of skipping a vaccine’s second dose are unknown
  • If a second dose is available, it must be taken for full protection
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And remember that you need to wear your mask after being vaccinated to halt the spread of Covid-19.


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