With COVID-19 vaccines approved for emergency use in several countries, sponsors of those vaccines and others under development must decide whether to continue their ongoing trials as is or risk damage to trial data by unblinding and giving participants in placebo groups the option to receive an approved vaccine.

Pfizer, the first sponsor with U.S. emergency-use approval, has chosen to offer the vaccine to eligible participants in the placebo group of its trial of BNT162b2, a decision that has raised concerns about the trial’s data integrity. Currently, healthcare personnel and long-term care facility residents are the only ones who can opt to receive the vaccine. Later, if the government decides to expand the eligibility requirements for who can receive the vaccine, Pfizer wants to make it available to those participants in its placebo group, the companies said.

Moderna appears to be taking a similar tack. With a COVID-19 vaccine candidate approved by the FDA for Emergency Use Authorization (EUA) on Friday, the company is discussing with the agency potential revisions to its phase 3 trial now that an EUA has been granted. Moderna plans to give trial participants the option of unblinding and offer the vaccine to all eligible patients in the placebo group.

AstraZeneca (AZ) may have more to lose if it allows unblinding, given the delays in its vaccine trials and the ongoing controversy over a reported dosing error that accidentally led to improved efficacy results. The company has provided guidance to trial investigators who will be responsible for overseeing the unblinding process.

Despite the effect unblinding would have on trial data, many sponsors, including Pfizer and AZ, cite an ethical responsibility to give participants in at least placebo control groups the option of receiving an approved vaccine that could protect them from the virus.