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The Oxford/AstraZeneca (ChAdOx1-S [recombinant] vaccine) COVID-19 vaccine: what you need to know

16 March 2022

Updated on 15 March 2022 pursuant to revised interim recommendations.

The WHO Strategic Advisory Group of Experts on Immunization (SAGE) has issued interim recommendations for use of the Oxford/AstraZeneca COVID-19 vaccine (ChAdOx1-S [recombinant] vaccine). 

You may access the guidance document here

Who should be vaccinated first?

While vaccine supplies are limited, it is recommended that priority be given to health workers at high risk of exposure and older people, including those aged 65 or older.

Countries can refer to the WHO Prioritization Roadmap and the WHO Values Framework as guidance for their prioritization of target groups.

Who else can take the vaccine?

Vaccination is recommended for persons with comorbidities that have been identified as increasing the risk of severe COVID-19, including obesity, cardiovascular disease, respiratory disease and diabetes. 

Although further studies are required for persons living with HIV or auto-immune conditions or who are immunocompromised, people in this category who are part of a group recommended for vaccination may be vaccinated after receiving information and counselling. 

Vaccination can be offered to people who have had COVID-19 in the past. But individuals may wish to defer their own COVID-19 vaccination for up to six months from the time of SARS-CoV-2 infection, to allow others who may need the vaccine more urgently to go first. If variants of concern are circulating, deferral could be shortened to 90 days.

Vaccination can be offered to breastfeeding women if they are part of a group prioritized for vaccination. WHO does not recommend discontinuation of breastfeeding after vaccination. 

Should pregnant women be vaccinated?

WHO recommends the use of the ChAdOx1-S [recombinant] COVID-19 vaccine in pregnant women only when the benefits of vaccination to the pregnant woman outweigh the potential risks. To help pregnant women make this assessment, they should be provided with information about the risks of COVID-19 in pregnancy, the likely benefits of vaccination in the local epidemiological context, and the current limitations of safety data in pregnant women. WHO does not recommend pregnancy testing prior to vaccination. WHO does not recommend delaying pregnancy or terminating pregnancy because of vaccination.

Who is the vaccine not recommended for?

People with a history of severe allergic reaction to any component of the vaccine should not take it.

The vaccine is not recommended for persons younger than 18 years of age pending the results of further studies.

What is the recommended dosage?

The recommended dosage is two doses given intramuscularly (0.5ml each) with an interval of 8 to 12 weeks. 

For countries that have not yet achieved high vaccine coverage rates in the high-priority groups and that are experiencing high incidence of COVID-19 cases combined with vaccine supply constraints, longer intervals i.e. up to 16 weeks can be considered. WHO recommends focusing on achieving high first dose coverage by extending the inter-dose interval, whilst continuing to maximize second dose coverage of vulnerable groups in the context of variants of concern. It should be noted that the full two dose regimen of this vaccine is believed to be more protective against variants of concern than a single dose alone.

Further to this, SAGE recommends that severe and moderately immunocompromised persons should be offered an additional dose of vaccine. This is due to the fact that this group is less likely to respond adequately to vaccination following a standard primary vaccination series and are at higher risk of severe COVID-19 disease.

Is a booster dose recommended for this vaccine?

A booster dose may be considered 4 – 6 months after completion of the primary vaccination series, though this is mainly recommended for the higher priority-use groups, in accordance with the WHO Prioritization Roadmap.

The benefits of booster vaccination are recognized following increasing evidence of waning vaccine effectiveness against mild and asymptomatic SARS-CoV-2 infection over time.

The need for, and timing of, booster doses for children aged 5-11 years has not yet been determined.

Can this vaccine be ‘mixed and matched’ with other vaccines?

SAGE accepts two heterologous doses of WHO EUL COVID-19 vaccines as a complete primary series.

For countries considering heterologous schedules, WHO has made recommendations to ensure equivalent or favourable immunogenicity or vaccine effectiveness for heterologous versus homologous schedules:

  • The Astra-Zeneca vaccine recommendations have also been modified to indicate that either of the mRNA COVID-19 vaccines (Pfizer or Moderna) can be used as a second dose following a first dose with the AstraZeneca vaccine dependant on product availability.
  • The Astra-Zeneca vaccine can be used as a second dose following any WHO EUL COVID-19 inactivated vaccines (Sinopharm, Sinovac or Bharat) dependant on product availability. 

When considering booster vaccination after having been vaccinated with ChAdOx1-S [recombinant] COVID-19 [Astra Zeneca] vaccine for the primary series, SAGE considers using a different type of COVID-19 vaccine for a third dose a more favourable option. However, maintaining a homologous schedule is still acceptable. 

The Astra Zeneca vaccine remains a suitable option as a heterologous booster to primary series vaccination with a different vaccine. 

Is it safe?

A very rare new type of adverse event called Thrombosis with Thrombocytopenia Syndrome (TTS), involving unusual and severe blood clotting events associated with low platelet counts, has been reported after vaccination with this vaccine. In countries with ongoing SARS-CoV-2 transmission, the benefit of vaccination in protecting against COVID-19 far outweighs the risks.

Guillain-Barré syndrome (GBS) has been reported very rarely following vaccination. However, a causal relationship with the vaccine has neither been confirmed nor ruled out and more rigorous studies are needed to fully assess the significance of these events. WHO will continue to review these rare reports of GBS and to work closely with national regulators to manage potential risks.

The Global Advisory Committee on Vaccine Safety, a group of experts that provides independent and authoritative guidance to the WHO on the topic of safe vaccine use, receives and assesses reports of suspected safety events of potentially international impact. 

How efficacious is the vaccine?

The vaccine efficacy against symptomatic SARS-CoV-2 infection was 76%. But this is specific to events from 15 days past second dose, with an inter dose interval of 29 days.

How does this compare with other vaccines in use?

It is impossible to compare vaccines head-to-head due to the different approaches taken in designing the respective studies, but overall, all of the vaccines that have achieved WHO Emergency Use Listing are highly effective in preventing severe disease and hospitalization due to COVID-19.

Does it work against new variants?

SAGE has reviewed all available data on the performance of the vaccine in the settings of variants of concern. SAGE currently recommends its use according to the WHO Prioritization Roadmap, even if virus variants are present in a country. Countries should assess the risks and benefits taking into consideration their epidemiological situation.

Preliminary findings highlight the urgent need for a coordinated approach for surveillance and evaluation of variants and their potential impact on vaccine effectiveness. As new data become available, WHO will update recommendations accordingly. There are no data yet for Omicron.

Does it prevent infection and transmission?

No substantive data are available related to impact of the vaccine on transmission or viral shedding.

In the meantime, we must maintain and strengthen public health and social measures that work: masking, physical distancing, handwashing, respiratory and cough hygiene, avoiding crowds, and ensuring good ventilation.

As of September 2021, the AstraZeneca vaccine is safe and effective at protecting people from the extremely serious risks of COVID-19, including death, hospitalization and severe disease. Read the 16 April 2021 statement of the WHO Global Advisory Committee on Vaccine Safety on AstraZeneca COVID-19 vaccine for reports of very rare side effects. The Council for International Organizations of Medical Sciences classifies rates of adverse events or medicines and vaccines as follows:

  • Very common > 1/10 
  • Common (frequent) > 1/100 and < 1/10 
  • Uncommon (infrequent) >1/1000 and < 1/100 
  • Rare> 1/10000 and <1/1000 
  • Very rare < 1/10000 


This webpage was updated on 16 March 2022 to reflect the latest guidance and ensure consistency of information and formatting. 

This webpage was updated on 5 January 2022 to update the latest guidance and ensure consistency of information and formatting.