Nov 18 (Reuters) - Many countries are rolling out COVID-19 vaccine booster shots, but there is no consensus among scientists that they are necessary and the World Health Organization wants the most vulnerable people worldwide to be fully vaccinated first. read more Here are some of the options countries and regions are considering: NORTH AMERICA The U.S. Centers for Disease Control and Prevention (CDC) on Sept. 24 backed a booster shot of the Pfizer (PFE.N) and BioNTech (22UAy.DE) vaccine for Americans aged 65 and older, some adults with underlying medical conditions and some adults in high-risk working and institutional settings. Canada"s National Advisory Committee on Immunization is recommending booster shots of an authorized mRNA vaccine to those who are moderately or severely immunocompromised. EUROPE The European Union"s drug regulator recommended on Oct. 4 a third dose of Pfizer-BioNTech and Moderna mRNA vaccines for those with a severely weakened immune system, but left it to member states to decide whether the wider population should get a booster. read more Recent contracts with Pfizer/BioNTech and Moderna (MRNA.O) have included the potential for the bloc to buy booster shots. These European countries are offering boosters to adults after they receive a full dose of a vaccine: ** Austria; Czech Republic; Germany (all over-18s); Hungary; Italy (Pfizer or Moderna booster shots for all inoculated, and for all those who have received a single dose of the Johnson & Johnson (JNJ.N) vaccine; Malta (all over-12s); Norway; Poland; Russia; Romania (only boosters from Pfizer or Moderna were approved); Serbia; Slovakia; Spain (Pfizer or Moderna booster shots for all inoculated with J&J) These countries are offering boosters to people with weak immune systems, the elderly or vulnerable: ** Belgium (mRNA); Bulgaria; Britain (plans to extend rollout to over-40s from over-50s); Denmark; Finland (may expand to other Finns later in the autumn); France (so far available only to over-65s, will be offered to over-50s from December); Ireland; Italy (will offer to all over-40s from Nov. 22); Lithuania; Netherlands (over-60s); Portugal; Slovenia; Spain (expanded from over-70s to over-60s); Sweden (larger population to get a jab in 2022); Switzerland (new recommendation for people under 65 expected in the next few weeks) AFRICA ** Morocco, which has administered the most doses in Africa, started giving a third dose beginning in October. ** Tunisia (5 months after second dose) ** Nigeria is trying to secure booster shots, the head of the presidential steering committee on COVID-19 said on Nov 15. ASIA, MIDDLE EAST These countries are offering boosters to adults after they receive a full dose: ** Bahrain (Sputnik V, all over-18s at least six months after second dose); Cambodia (AstraZeneca); Indonesia (health workers only, wider population planned in 2022); Israel (all over-12s); South Korea (initial doses to high-risk groups or people with weakened immune systems); Turkey; Thailand (AstraZeneca or mRNA-type booster shots to people who were administered Sinovac brand); UAE (mandatory for people inoculated with Sinopharm vaccine) These countries are offering boosters to people with weak immune systems, the elderly or vulnerable: ** China; Hong Kong; Singapore AUSTRALIA began administering Pfizer COVID booster shots in November to fully vaccinated adults. LATIN & CENTRAL AMERICA These countries are offering boosters to adults after they receive a full dose: ** Brazil; Uruguay (offers a Pfizer dose for those fully vaccinated with Sinovac) These countries are offering boosters only to the immunosuppressed: ** Ecuador, Panama These are offering boosters to elderly, vulnerable and at-risk people: ** Chile; Brazil; Dominican Republic, El Salvador (elderly, health workers, people with underlying health conditions) COMPANIES The U.S. Food and Drug Administration authorized on Sept. 22 a booster dose of the Pfizer and BioNTech COVID-19 vaccine for those 65 and older, all people at high risk of severe disease, and others who are regularly exposed to the virus. read more The FDA authorized a third dose by Pfizer-BioNTech and Moderna Inc on Aug. 13 for people with compromised immune systems. Pfizer/BioNTech, as well as Moderna, have also requested FDA approval for the use of their booster shots for all adults. [nL4N2S83HI] The FDA cleared in October booster shots of vaccines from Moderna and J&J. read more The European Union"s drug regulator (EMA) authorised the use of Moderna and Pfizer COVID-19 booster shots for over-18s, at least six months after the second dose. read more EMA is also considering an authorisation for AstraZeneca"s booster dose. read more Canada authorised booster dose of Moderna and Pfizer vaccines for adults. read more Compiled by Dagmarah Mackos, Veronica Snoj, Filipe Braganca; Editing by William Maclean, Giles Elgood and Mark Heinrich