{"id":1150,"date":"2021-02-19T08:29:20","date_gmt":"2021-02-19T13:29:20","guid":{"rendered":"https:\/\/covidqanda.org\/?p=1150"},"modified":"2021-02-19T08:29:20","modified_gmt":"2021-02-19T13:29:20","slug":"does-vaccine-efficacy-mean-what-i-think-it-means","status":"publish","type":"post","link":"https:\/\/covidqanda.org\/?p=1150","title":{"rendered":"Does vaccine efficacy mean what I think it means?"},"content":{"rendered":"\n

Question:<\/span><\/strong>\u00a0We keep talking\u00a0about vaccine efficacy.\u00a0 I thought 95% efficacy meant that 95 of 100 people vaccinated would be protected.\u00a0 Is that correct?<\/p>\n\n\n\n

Answer:\u00a0<\/span><\/strong>That’s what a lot of us think vaccine\u00a0efficacy\u00a0means, but alas, it’s incorrect.\u00a0 Instead of thinking about vaccine efficacy in terms of “people protected,”\u00a0we should think about it in terms of “risk reduction.”\u00a0 Pfizer had 95% efficacy and Moderna had 94% efficacy in clinical trials.\u00a0 This means that the two vaccines reduced the vaccinated population’s risk of becoming sick with COVID by 95% and 94% respectively.\u00a0\u00a0<\/p>\n\n\n\n

Two days ago, Lancet<\/em>\u00a0published a short comment on this very topic, What does 95% COVID-19 vaccine efficacy really mean?<\/a>.\u00a0 As the author describes, if we estimate that the virus has an attack rate of 1% over a 3 month period, a non-vaccinated population would expect to experience 1,000 COVID cases per every 100,000 individuals.\u00a0 If that population were vaccinated with a 95% effective vaccine, we would expect a 95% reduction in the risk of disease; instead of 1,000 cases\/100,000 individuals over 3 months, we would expect 50 cases\/100,000 individuals over 3 months.<\/p>\n\n\n\n

A quick note on vaccine efficacy vs. effectiveness:<\/em><\/strong>\u00a0 Vaccine efficacy is a measure of how well the vaccine performs in clinical settings.\u00a0 Meanwhile, vaccine effectiveness is a measure of how well the\u00a0vaccine prevents disease in “real world” settings, e.g. outside of clinical trials.\u00a0 The data from Israel shared in\u00a0yesterday’s Q&A\u00a0<\/a>was vaccine effectiveness; preliminary analytic results show Pfizer’s vaccine effectiveness (94%) is nearly equivalent to its efficacy (95%).\u00a0\u00a0<\/p>\n","protected":false},"excerpt":{"rendered":"

Question:\u00a0We keep talking\u00a0about vaccine efficacy.\u00a0 I thought 95% efficacy meant that 95 of 100 people vaccinated would be protected.\u00a0 Is that correct? Answer:\u00a0That’s what a lot of us think vaccine\u00a0efficacy\u00a0means, but alas, it’s incorrect.\u00a0 Instead of thinking about vaccine efficacy in terms of “people protected,”\u00a0we should think about it in terms of “risk reduction.”\u00a0 Pfizer […]<\/p>\n","protected":false},"author":1,"featured_media":1151,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"nf_dc_page":"","om_disable_all_campaigns":false,"_monsterinsights_skip_tracking":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_feature_clip_id":0,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_post_was_ever_published":false},"categories":[17,64],"tags":[175,174],"class_list":["post-1150","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-risk-reduction","category-vaccines","tag-effectiveness","tag-efficacy","fallback-thumbnail"],"jetpack_sharing_enabled":true,"jetpack_featured_media_url":"https:\/\/i0.wp.com\/covidqanda.org\/wp-content\/uploads\/2021\/02\/pexels-photo-4167541.jpeg?fit=867%2C1300&ssl=1","_links":{"self":[{"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/posts\/1150","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/covidqanda.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1150"}],"version-history":[{"count":1,"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/posts\/1150\/revisions"}],"predecessor-version":[{"id":1152,"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/posts\/1150\/revisions\/1152"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/covidqanda.org\/index.php?rest_route=\/wp\/v2\/media\/1151"}],"wp:attachment":[{"href":"https:\/\/covidqanda.org\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1150"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/covidqanda.org\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1150"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/covidqanda.org\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1150"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}