The two doses are likely to be required because SARS-CoV-2 is a newly emerged virus that no one has developed antibodies against. Also, with many potential vaccines being created using new systems it’s believed two doses will be required for full immunity.

The first shot would prime the immune system, helping it recognize the virus. The second shot would strengthen the immune response.

Of the more than 100 vaccine candidates in various stages of testing, almost all are expected to be a two-dose regime, said Barry Bloom, an immunologist and professor of public health at the Harvard T.H. Chan School of Public Health in Boston…

Any coronavirus vaccine that requires two doses will probably need to be given about a month or two apart, though that’s still being worked out, said L.J. Tan, chief strategy officer with the Immunization Action Coalition and co-chair of the National Adult Immunization Summit and National Influenza Vaccine Summit.