Which Covid vaccine is the best? Which Covid vaccine is the most effective? How do we measure vaccine effectiveness? Do Covid vaccines work against new variants? Is it possible that Covid vaccines may make things worse in the future? I answer all of these questions in this video. First, we compare the seven vaccines that have published phase 3 trial data as of June 2021. Those include Pfizer, Moderna, Sputnik, Johnson & Johnson, Sinopharm, and AstraZeneca. I look at the phase 3 trial data published in peer-reviewed journals. I compare the vaccine effectiveness between each vaccine. I also look at the differences in the phase III studies and see that the SARS-CoV-2 variants probably explain the differences in the vaccines. Each vaccine becomes less effective against variants that have more mutations, especially mutations on the spike protein, such as the E484K mutation, which can cause immune escape. Important mutations discussed include the South African variant, B.1.351, the Brazil variant, P.1, the UK variant, B.1.1.7, and the Indian variant, B.1.167. Finally, I explain why this matters. Countries, such as Thailand, Vietnam, and Malaysia, that have not yet had widespread Covid outbreaks may not be able to effectively use vaccines to prevent future outbreaks, as the current vaccines have substantially reduced effectiveness against the variants in those countries. I also explore the possibility that vaccines may initiate antibody-dependent enhancement, or ADE, which may make future infections worse for some people. 0:00 Introduction 0:53 How do we measure the effectiveness of a vaccine? 2:13 Which Covid vaccines have published phase 3 trial data? 3:36 How effective were Covid vaccines in phase 3 trials? 5:48 How much do virus variants reduce vaccine efficacy? 9:46 Not all effectiveness results can be compared 10:30 The most vaccinated country had a Covid outbreak 12:11 Possibility of future antibody-dependent enhancement For further reading: Effect of 2 Inactivated SARS-CoV-2 Vaccines on Symptomatic COVID-19 Infection in Adults A Randomized Clinical Trial | https://doi.org/10.1001/jama.2021.8565 Efficacy and Safety of the mRNA-1273 SARS-CoV-2 Vaccine | https://doi.org/10.1056/NEJMoa2035389 Safety and Efficacy of the BNT162b2 mRNA Covid-19 Vaccine | https://doi.org/10.1056/NEJMoa2034577 Safety and Efficacy of Single-Dose Ad26.COV2.S Vaccine against Covid-19 | https://doi.org/10.1056/NEJMoa2101544 Safety and efficacy of an rAd26 and rAd5 vector-based heterologous prime-boost COVID-19 vaccine: an interim analysis of a randomised controlled phase 3 trial in Russia | https://doi.org/10.1016/S0140-6736(21)00234-8 Safety and efficacy of the ChAdOx1 nCoV-19 vaccine (AZD1222) against SARS-CoV-2: an interim analysis of four randomised controlled trials in Brazil, South Africa, and the UK | https://doi.org/10.1016/S0140-6736(20)32661-1 Effectiveness of COVID-19 vaccines against the B.1.617.2 variant | https://khub.net/documents/135939561/430986542/Effectiveness+of+COVID-19+vaccines+against+the+B.1.617.2+variant.pdf/204c11a4-e02e-11f2-db19-b3664107ac42 Effectiveness of the BNT162b2 Covid-19 Vaccine against the B.1.1.7 and B.1.351 Variants | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8117967/pdf/NEJMc2104974.pdf Antibody-dependent enhancement and SARS-CoV-2 vaccines and therapies | https://www.nature.com/articles/s41564-020-00789-5.pdf Antibody-dependent Enhancement (ADE) and Vaccines | https://www.chop.edu/centers-programs/vaccine-education-center/vaccine-safety/antibody-dependent-enhancement-and-vaccines