Please make sure your browser supports JavaScript and cookies and that you are not blocking them from loading. The thinking behind it is different vaccines provoke different parts of the immune system  the main substances being antibodies and T cells. But, because they work in different ways, getting doses of different jabs could 'maximise' the immune response and give better, longer lasting protection. While the UK leaving Europe adds 'complexity' to the process, robust plans are in place to mitigate Brexit. Why did this happen?
The u. s. government's operation['s. a.l.t.ismewar.quiet.com':
( z0 operate :ARG0 ( z1 government-organization :ARG0-of ( z2 govern :ARG1 ( z3 country :name ( z4 name :op "U.S." ) ) ) ) :location ( z5 url-entity :ARG1-of ( z6 equal ) ) )
(z0 / operate-01
    :ARG0 (z1 / government-organization
              :ARG0-of (z2 / govern-01
                           :ARG1 (z3 / country
                                     :name (z4 / name
                                               :op1 "U.S."))))
    :location (z5 / url-entity
                  :ARG1-of (z6 / equal-01)))
The u. s. government 's operation [ 's . a . l . t . ismewar . quiet . com ' :
z3 :name z4
z4 :op1 0
z1 :ARG0-of z2
z2 :ARG1 z3
z0 :ARG0 z1
z0 :location z5
z5 :ARG1-of z6