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.