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

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?

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.

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.

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’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.

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.

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.

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