The AstraZeneca dose interval has been reduced from 12 weeks to 9 weeks, but should we accept that or try to delay to 12 weeks or later?
This is not a straightforward decision, due to the threat posed by the new Delta variant.
Take a look at the pros and cons, and see which is better for you!
AstraZeneca Vaccine : Dose Interval Reduced To 9 Weeks!
This is likely due to the surge of cases caused by the highly-infectious Delta strain of COVID-19, which has been shown to infect people within seconds of close contact.
A single dose of the AstraZeneca vaccine has been shown to offer very limited protection against the Delta variant, so it is critical for people to get their second dose quickly.
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AstraZeneca Vaccine : Accept Or Delay 9 Week Dose Interval?
Now, many people have asked me – should we accept the new 9 week dose interval, or should we try to delay the second dose to the full 12 weeks or later?
Unfortunately, the answer is not quite so straightforward, due to the threat posed by the Delta variant.
You have to weigh the higher efficacy from waiting, against the risk of getting infected by a COVID-19 variant before getting the second dose.
Here are some factors for us to take into consideration (please see the next section for the evidence / proof) :
Risk of Exposure
If you are working from home, and have very little to no contact with strangers, you can risk waiting the full 12 weeks.
However, if your work requires you to be in close contact with other people, you should get the second dose as early as you can.
Age + Health
If you are a healthy young adult, your risk of being hospitalised with COVID-19 will be low since you are protected by the first AstraZeneca vaccine dose. You can risk waiting the full 12 weeks.
However, if you are a senior citizen or have one or more co-morbidities, you should get the second dose as early as you can.
Vulnerable Family Members
If other members of your family living in the same home received one or both vaccine doses, you can risk waiting the full 12 weeks.
However, if you have vulnerable family members (who cannot or are not vaccinated yet), you should get the second dose as early as you can.
AstraZeneca Vaccine Dose Interval : Evidence For / Against 9 Weeks
For a better understanding of the pros and cons of increasing or reducing the AstraZeneca vaccine dose interval, let me take you through my research and reasoning…
My analysis above was based on :
- a pooled analysis of four studies covering 24,422 participants, published in The Lancet on 19 February 2021.
- 22 May 2021 and 14 June 2021 data from Public Health England.
- a preprint paper looking at ultra-long dose intervals involving 321 participants, published in The Lancet on 28 June 2021
Single Dose Efficacy
A single dose of the AstraZeneca vaccine proved to be very efficacious against symptomatic COVID-19.
After 21 days, a single dose offered up to 78.3% efficacy 90 days after vaccination.
This is the first reason why WHO recommended that the second AstraZeneca dose be given 8 to 12 weeks after the first dose.
|Time After 1 Dose||Efficacy||Efficacy Range|
|22 to 30 days||76.7%||47.0% to 89.8%|
|31 to 60 days||72.8%||32.9% to 89.0%|
|22 to 90 days||76.0%||59.3% to 85.9%|
|61 to 90 days||78.3%||36.4% to 92.6%|
|91 to 120 days||31.6%||-141.8% to 80.7%|
Change In Dose Interval
The AstraZeneca vaccine was originally designed with a 21-day interval between Dose 1 and Dose 2.
However, this study showed that the AstraZeneca vaccine’s efficacy increased with longer delays between Dose 1 and Dose 2!
This is the second reason why WHO recommended that the second AstraZeneca dose be given 8 to 12 weeks after the first dose.
As you can see, the efficacy of the AstraZeneca vaccine at 9 weeks is significantly lower than its efficacy at 12 weeks – 63.7% vs. 81.3% – a 27.6% difference in efficacy.
After 14 days
|< 6 Weeks||55.1%||33.0% to 69.9%|
|6 – 8 Weeks||59.9%||32.0% to 76.4%|
|9 – 11 Weeks||63.7%||28.0% to 81.7%|
|≥ 12 Weeks||81.3%||60.3% to 91.2%|
At this point, you may be wondering – why is the single dose efficacy much higher than the two-dose efficacy?
The two results are not directly comparable and should be looked at separately, because :
- the single-dose cases were from much earlier in the year than the double-doses cases, when the strength of the epidemics were different.
- the single dose efficacy was based on cases 21 days or later after vaccination, while the double-dose efficacy was based on cases 14 days or later after vaccination.
Ultra-Long Dose Intervals Work Even Better
The preprint paper in the Lancet appears to back-up the evidence that longer dose intervals improve the AstraZeneca vaccine’s efficacy.
It showed that longer dose intervals like 15-25 weeks and even 44-45 weeks were associated with higher antibody response when Dose 2 is administered!
Efficacy Against Alpha + Delta Variants
Now, it is important to understand why health authorities are speeding up the second dose.
Data from Public Health England showed that a single AstraZeneca vaccine dose is only partially protective against the Alpha variant, and is particularly weak against the Delta variant.
The good news is – the second dose boosted immunity, offering robust protection against both variants.
|1 Dose||2 Doses||1 Dose||2 Doses|
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