Study: Vaccine Effectiveness against Infection Drops to Practically Nothing after >7 Months

A pre-print study from Sweden by P. Nordström et al titled “Effectiveness of Covid-19 vaccination against risk of symptomatic infection, hospitalization, and death up to 9 months: a Swedish total-population cohort study” finds that vaccine effectiveness wanes somewhat consistently to other studies.


The study compromised of 842,974 matched pairs of fully vaccinated and unvaccinated individuals. It found…

…there was a progressive waning in vaccine effectiveness (2 doses of any vaccine) against symptomatic infection over time. Effectiveness peaked at day 15-30 (92%; 95% CI, 91-93, P<0·001) and declined marginally at day 31-60 (89%; 95% CI, 88-89, P<0·001). From thereon, the waning became more pronounced, and from day 211 days onwards, there was no remaining detectable effectiveness (23%; 95% CI, -2-41, P=0·07).

As for the vaccines used (Pfizer, Moderna and Astrazeneca):

Effectiveness of BNT162b2 [Pfizer] waned to 47% (95% CI, 39-55, P<0·001) at day 121-180, and no effectiveness was detected from day 211 and onwards (23%; 95% CI, -2-41, P=0·07). Waning was slightly slower for mRNA-1273 [Moderna], with a remaining effectiveness of 59% (95% CI, 18-79, P<0·001) after more than 180 days of follow up, and for heterologous ChAdOx1 nCoV-19 / mRNA [Astrazeneca] schedules (66%; 95% CI, 41-80, P<0.001 from day 121 and onwards). In contrast, there was no detectable effectiveness for homologous ChAdOx1 nCoV-19 from day 121 and onwards (-19%; 95% CI, -97-28, P=0·49).

Also vaguely consistent with other studies, the vaccine effectiveness against severe illness/hospitalization/death does not drop as dramatically as against symptomatic infection.

Vaccine effectiveness (any vaccine) was 89% at day 15-30 (95% CI, 83-93, P<0·001), which declined to 74% (95% CI, 47-87, P<0·001) by day 121-180, and from day 181 and onwards, there was no detectable associated effectiveness (42%; 95% CI, -35-75, P=0·21).

Taken at face value, the strengths of this study seem to be the relatively large sample size, and that it looks at data up to and beyond day 210. This is the closest thing to long-term that we have. However, some of the 95% CIs are quite wide. I also think it can present its results more clearly. Either way, it does generally show that vaccine effectiveness against infection drops dramatically within months.


Figure 2: Vaccine effectiveness % against symptomatic COVID-19 infection.
Figure 2: Vaccine effectiveness % against symptomatic COVID-19 infection.
 

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