<story><title>Pfizer 91% effective in updated data, protective against South African variant</title><url>https://www.reuters.com/article/idUSKBN2BO55Y</url></story><parent_chain><item><author>throwaway894345</author><text>I&amp;#x27;m always confused about &amp;quot;effectiveness&amp;quot; numbers. Are these &amp;quot;effective at preventing severe disease&amp;quot; or &amp;quot;effective at preventing symptomatic disease&amp;quot; or against transmission period? The article seems to suggest it&amp;#x27;s the latter, but it seems somewhat ambiguous. Does this term have a specific definition in epidemiology that journalists and pharma marketers assume everyone understands what kind of &amp;quot;effectiveness&amp;quot; they&amp;#x27;re talking about?</text></item></parent_chain><comment><author>gregwebs</author><text>Most of the responses to your question that I have read are factually incorrect in some way. Efficacy&amp;#x2F;effectiveness is up to each study to define.&lt;p&gt;Generally you should assume they are measuring symptoms of infections that go beyond the upper respiratory tract. This is because the vaccines provide little protection against an upper respiratory tract infection but instead are very effective at preventing it from progressing further into a more severe infection. This is referred to as non-sterilizing immunity: it is probably the most important concept for everyone to understand about the vaccines.&lt;p&gt;I am seeing a lot of comments here claim that vaccines are 100% effective at preventing death or severe infection where severe is generally defined as hospitalization. Nothing is 100% effective, and particularly not vaccines for those whose immune system does not respond properly. Death in particular is generally not an endpoint that can be compared in a statistically significant way in these studies. [1]&lt;p&gt;I will still be getting the first imperfect vaccine I am allowed to at the first chance I get. When I needed to get an appointment for my Mom I started writing a user script to help with that. [2]&lt;p&gt;[1] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;dalewharrison.substack.com&amp;#x2F;p&amp;#x2F;vaccine-boosterism&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;dalewharrison.substack.com&amp;#x2F;p&amp;#x2F;vaccine-boosterism&lt;/a&gt;&lt;p&gt;[2] &lt;a href=&quot;https:&amp;#x2F;&amp;#x2F;gist.github.com&amp;#x2F;gregwebs&amp;#x2F;265e0ef6b1a3051377cfc4d66acecf81&quot; rel=&quot;nofollow&quot;&gt;https:&amp;#x2F;&amp;#x2F;gist.github.com&amp;#x2F;gregwebs&amp;#x2F;265e0ef6b1a3051377cfc4d66ac...&lt;/a&gt;</text></comment>
<story><title>Pfizer 91% effective in updated data, protective against South African variant</title><url>https://www.reuters.com/article/idUSKBN2BO55Y</url></story><parent_chain><item><author>throwaway894345</author><text>I&amp;#x27;m always confused about &amp;quot;effectiveness&amp;quot; numbers. Are these &amp;quot;effective at preventing severe disease&amp;quot; or &amp;quot;effective at preventing symptomatic disease&amp;quot; or against transmission period? The article seems to suggest it&amp;#x27;s the latter, but it seems somewhat ambiguous. Does this term have a specific definition in epidemiology that journalists and pharma marketers assume everyone understands what kind of &amp;quot;effectiveness&amp;quot; they&amp;#x27;re talking about?</text></item></parent_chain><comment><author>amichal</author><text>My layman&amp;#x27;s understanding was that &amp;quot;91%&amp;quot; &amp;quot;effective at preventing disease&amp;quot; (from the TFA) goes something like this:&lt;p&gt;&lt;pre&gt;&lt;code&gt;  - Take two identical[1] populations of people
  - Wait for some people (100 for easy math here) in the unvaccinated group to get the disease. Probably measured by a PCR test indicating presence of the virus and ignoring symptoms since they say &amp;quot;preventing disease&amp;quot; and not severe symptoms, hospitalization, death etc. 
  - Count how many folks have the disease in the vaccinated group[2]. In the &amp;quot;91%&amp;quot; case we would expect 9 people to test positive, meaning 91 of 100 people we would expected to get the disease did not.
&lt;/code&gt;&lt;/pre&gt;
I like to think of it as I am ~1&amp;#x2F;10th as likely to get the disease as I would be if i had not been vaccinated.&lt;p&gt;Is this what they mean in this?&lt;p&gt;&lt;pre&gt;&lt;code&gt;  [1] Doing your best to create identical groups by controlling for population differences such as gender, age, and other known risk factors, 
  [2] We need also to control for time in the study etc.
&lt;/code&gt;&lt;/pre&gt;
[Lots of editing for formatting etc]</text></comment>