Covid US: Vaccine effectiveness against infection fell from 91% to 78% after Delta became dominant

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COVID-19 vaccine effectiveness against infection fell from 91% to 78% after the Delta variant became dominant, but remained above 90% against hospitalization and death, CDC report finds

  • A new CDC report looked at the effectiveness of COVID-19 vaccines before and after the Delta variant became the dominant strain in the U.S.
  • Prior to the variant’s dominance, the vaccines were 91% effective against infection, 92% effective against hospitalization and 90% effective against death
  • After Delta became dominant, effectiveness against infection fell to 78% but barely dropped against hospitalization and death at 91% and 90%, respectively 
  • CDC say this means that fully vaccinated are five times less likely to contract Covid, 10 times less likely to be hospitalized and 11 times less likely to die 










COVID-19 vaccine effectiveness fell against infection after the Delta variant became dominant in the U.S., but remained high against hospitalization and death. 

According to a new report published by the Centers for Disease Control and Prevention (CDC) on Friday, the shot’s effectiveness against contracting Covid fell from 91 percent in April to 78 percent in July.

However, the effectiveness against hospitalization and death due to the virus stayed above 90 percent. 

This means that, according to the CDC, fully vaccinated Americans are five times less likely to contract Covid, 10 times less likely to be hospitalized and 11 times more likely to die.

The report is part of a growing body of evidence that the vaccines became less effective after the highly transmissible Delta variant started circulating, but that they still protect recipients against the most severe outcomes linked to the virus.

A new CDC report found the COVID-19 vaccine's effectiveness against infection fell from 91% in April to 78% in July. Pictured: Number of cases per 100,000 among unvaccinated people (black line) versus vaccinated people (blue line) from April to July

A new CDC report found the COVID-19 vaccine’s effectiveness against infection fell from 91% in April to 78% in July. Pictured: Number of cases per 100,000 among unvaccinated people (black line) versus vaccinated people (blue line) from April to July

The effectiveness against hospitalizations barely fell from 92% to 90% effective and barely fell against from 94% to 91%. Pictured: Number of hospitalizations per 100,000 among unvaccinated people (black line) versus vaccinated people (blue line) on left; deaths per 100,000 among unvaccinated versus vaccinated on right

The effectiveness against hospitalizations barely fell from 92% to 90% effective and barely fell against from 94% to 91%. Pictured: Number of hospitalizations per 100,000 among unvaccinated people (black line) versus vaccinated people (blue line) on left; deaths per 100,000 among unvaccinated versus vaccinated on right

For the report, the team looked at COVID-19 cases, hospitalizations and deaths in 13 U.S. jurisdictions between April 4 and July 17.

The study period was split in two: from April 4 to June 19, before the Delta variant was dominant; and June 19 to July 17, after the Delta variant made up more than 50 percent of all cases.

During the pre-Delta dominance period, fully vaccinated Americans made up five percent of Covid cases, seven percent of hospitalizations and eight percent of deaths.

According to the report, 10.1 per 100,000 vaccinated people fell ill, 0.7 per 100,000 were hospitalized and 0.1 per 100,000 died. 

This equates to the vaccines being 91 percent effective against infection, 92 percent effective against hospitalization and 90 percent effective against death.

However, after Delta became dominant, fully vaccinated people made up 18 percent if all cases, 14 percent of hospitalizations and and 16 percent of deaths.

‘With 53 percent [vaccination] coverage reported during June 20–July 17, vaccinated persons were expected to account for 10 percent of cases at a constant [vaccine effectiveness] of 90 percent,’ the authors wrote.

‘The observed 18 percent would have been expected at a lower vaccine effectiveness of 80 percent.’

This means that the efficacy of the Covid vaccines against infection fell to 78 percent during summer 2021.

There was a dramatic increase from 10.1 per 100,000 fully vaccinated people contracting COVID-19 in early 2021 to 19.4 per 100,000 in late 2021. 

However, there were no dramatic increases in the rate of vaccinated people being hospitalized or dying from Covid after Delta became dominant and held steady at 0.7 per 100,000 and 0.1 per 100,000, respectively. 

This means the vaccines were still 90 percent effective against hospitalization nd 91 percent against death. 

‘Getting vaccinated protects against severe illness from COVID-19, including the Delta variant,’ the authors concluded.  

‘Monitoring COVID-19 incidence by vaccination status might provide early signals of potential changes in vaccine effectiveness that can be confirmed through robust controlled studies.’  

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