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The Delta Variant Dominates

As can be seen from the chart above, the delta variant is now totally dominate in the US. The picture is similar in other countries, with the UK first seeing the variant in April 2021 and by late June, 99% of sequenced cases were Delta. The critical question now is:

Are the current covid vaccines effective against the Delta variant?

Research from Oxford University just published and available as a pre-print here, goes some way to show how effective two common Covid19 vaccines are (Pfizer and AstraZeneca).  Over 350,000 patients PCR results were examined, during 2020, when the Alpha variant was dominant and another ~350,00 again in 2021, when the Delta variant had arrived. The patients were not the same both times, rather just the available data was used.

The summary is, yes, they are still effective, but important differences and changes are noted.


Initial Protection

Pfizer starts out at 94% effective against Alpha, which drops to 88% for Delta.  AstraZeneca goes from 75% to 67% effective. So both maintain good protection.


Protection wanes over Time

The study tried to determine how protection changes over time.  The estimates arrived at were that the Pfizer vaccine reduces in effectiveness by around 22% each month and AstraZeneca by 7%.  This means that after about 5 months, the effectiveness of both, is around the same.


Risk to infect others

Another part of the study looked at the viral load seen in the test samples. This is an indication of how infectious a person is. Here a large difference was discovered. The viral load for the Delta variant was higher and importantly, was similar for patients regardless of their vaccination status.  Therefore, the vaccines could be less effective at stopping transmission, compared to the Alpha variant.  The flip side to this however, is to remember that the vaccines stop most infections in the first instance.

A question mark still remains over the infectiousness of a vaccinated person and unvaccinated with the same viral load. More studies are required to answer this point.



The Pfizer and AstraZeneca vaccines offer good protection against the Delta variant. It declines over time and protection weakens faster in older age groups, all pointing to the need for booster jabs for the over 50s, especially if further highly infectious variants appear.

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