Vaccine efficacy and duration of protection

Are COVID-19 vaccines effective against new variants of concern of the virus?

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Vaccine efficacy against severe disease remains for all variants of concern, despite some drops in vaccine efficacy against symptomatic or asymptomatic disease.

All viruses, including SARS-CoV-2, the virus that causes COVID-19, change over time. Although most changes have little to no impact on the virus’ properties, some may affect how easily it spreads, the associated disease severity, or the performance of vaccines, therapeutic medicines, diagnostic tools, or other public health and social measures.

Variants that pose an increased risk to global public health are defined by WHO as variants of interest (VOIs) or variants of concern (VOCs). A Variant of Interest can become a Variant of Concern if it proves to be a greater threat as demonstrated by international spread, greater disease severity, immune escape or ability to out-compete other strains. The classification of viruses enables the global community to prioritize monitoring and research, and ultimately to inform the ongoing response to the COVID-19 pandemic.

The following have been classified as variants of concern:

  • The Alpha variant (B.1.1.7) is known to increase viral transmissibility and was previously the predominant variant in Europe. This variant has been described as having little escape from previous immunity.
  • The Beta (B.1.351) variant is less easily neutralized by convalescent plasma obtained from patients infected with previous variants and preliminary evidence suggests reduced efficacy of some vaccines against mild to moderate disease;
  • The Gamma (P.1) variant can cause severe disease even in people who have been previously infected, although this information needs to be expanded with further studies. Similarly, moderate escape from the immune response has been described with this variant.
  • The Delta (B.1.617.2) variant has shown in animal studies, which have modelled pathogenesis in humans, higher viral loads and greater shedding. Reductions in vaccine neutralizing titres against new VOCs has been observed, although with the Delta variant these decreases are not as great as with some other variants. Immunology of vaccine protection is complex, and although protection may be predicted by antibody responses as measured directly after vaccination, we don’t have an absolute correlate, we don’t know the role of anamnestic response, the contribution of T-cells, or the vaccine platform dependence on the efficacy. For all these reasons we have to be very careful in the interpretation of the results of studies assessing the impact of a VOC on vaccine protection. However, randomized data are lacking for the Delta variant with most vaccines.

Rapid development of efficacious COVID-19 vaccines is one of the few true success stories from this pandemic. However, virus variants will continue to flourish as long as virus transmission is ongoing and may therefore threaten the progress made so far and prolong the pandemic. That’s why continuous monitoring of the clinical, molecular and vaccine-related behaviour of the virus is a priority.

We must remain vigilant and not let down our guard down. While expanding vaccination and making vaccines available to those at highest risk we must continue public health and social measures, like wearing masks, frequently washing hands and social distancing where needed.


  • <https://www.nejm.org/doi/full/10.1056/NEJMsr2105280#article_references
  • <https://www.mscbs.gob.es/en/profesionales/saludPublica/ccayes/alertasActual/nCov/documentos/COVID19_Actualizacion_variantes_20210705.pdf>


  • Vaccines remain very effective in preventing severe disease, caused by circulating virus variants.
  • Variants will continue to emerge as long as the virus circulates.
  • Both vaccination and public health and social measures to suppress transmission are needed to control the pandemic.