The current pause on administering Johnson & Johnson’s (J&J) one-dose COVID-19 vaccine likely will continue in the U.S. after the CDC’s Advisory Committee on Immunization Practices (ACIP) declined to make a recommendation on a path forward April 14.

Instead, ACIP said more information is needed to define who’s at risk of rare blood clotting events after receiving the vaccine and to better assess the risk-benefit profile. But the group stopped short of voting to recommend a continued halt, with some members expressing concern about the impact that could have on vulnerable populations worldwide as COVID-19 infections surge.

One member noted that the pause, although only imposed April 13, already has been devastating for frontline health care workers who were looking to use it with homebound patients and other vulnerable populations. Others worried about the global inequities that could arise.

Stressing the urgency of evaluating the situation as quickly as possible, ACIP member Beth Bell said she didn’t want the committee’s nondecision “to send the message that there’s some huge concern here or that there’s something fundamentally wrong” with the J&J vaccine. She stressed that the blood clotting events were “very rare,” adding that “nothing in life is risk-free.”

After six reported cases of cerebral venous sinus thrombosis (CVST) with thrombocytopenia occurring seven to 14 days after vaccination with J&J’s adenovirus vaccine, the FDA and CDC announced the pause as they issued a health alert to providers and consumers to make them aware of the events that can be fatal when treated with heparin, the normal drug used for blood clots

At the time of that announcement, acting FDA Commissioner Janet Woodcock said she expected the pause to last a couple of days. But if the CDC follows the comments made in the ACIP discussion, the pause could last a few weeks as more data is collected and the investigation continues into the six known incidents.

With 3.7 million doses of the J&J vaccine administered within the past two weeks, more incidents could be reported soon, providing the data ACIP wants to make a more informed decision. Committee members noted that although the six cases reported to date involved white women aged 18 to 48, there was a similar incident reported during the vaccine’s clinical trial last year that occurred in a 25-year-old male. Also, 106 cases have been reported in a broader population in the EU and U.K. in connection with Astrazeneca plc’s COVID-19 vaccine, which also is an adenovirus vaccine.

(While the J&J vaccine has been authorized in the EU, it has not yet been used there. The Astrazeneca vaccine is still undergoing review in the U.S.)

Advantage mRNA

The concerns about the adenovirus vaccines are tipping the scales toward the new mRNA vaccines, such as those produced by Pfizer Inc.-Biontech SE and Moderna Inc. While there have been three CVST cases reported in people receiving Moderna’s vaccine, none of them was accompanied by thrombocytopenia. And no incidents have been reported in connection with the Pfizer-Biontech vaccine, even though millions of doses have been administered.

In looking ahead to the potential need for boosters and vaccines targeting COVID-19 variants, European Commission (EC) President Ursula von der Leyen said April 14 the EU will “need to focus on technologies that have proven their worth; mRNA vaccines are a clear case in point.”

With that in mind, the EC is negotiating a third contract with Pfizer-Biontech for an additional 1.8 billion doses of their COVID-19 vaccine to be delivered from 2021 to 2023. The contract requires the production of the vaccines, and all essential components, to be based in the EU.

Meanwhile, a day after Russian officials avoided questions about the safety of the Sputnik V adenovirus vaccine in light of the concerns about the Astrazeneca and J&J products, the Moscow-based Gamaleya National Centre of Epidemiology and Microbiology, which developed Sputnik, issued a statement to differentiate it from the other two adenovirus vaccines.

The three vaccines “based on adenoviral vectors have significant differences in their structure and production technology. Therefore, there is no reason and no justification to extrapolate safety data from one vaccine to safety data from other vaccines,” the center said. It added that a comprehensive analysis of adverse events from Sputnik clinical trials and over the course of mass vaccinations showed no cases of cerebral venous sinus thrombosis.

The center pointed out that Astrazeneca’s vaccine uses chimpanzee adenovirus to deliver the antigen, which consists of S-protein combined with leader sequence of tissue-type plasminogen activator, while the J&J vaccine uses human adenovirus serotype Ad26 and full-length S-protein stabilized by mutations. Sputnik V is a two-component vaccine in which adenovirus serotypes 5 and 26 are used, and the antigen insert is an unmodified full-length S-protein.

Another difference the Gamaleya Center noted is that the J&J vaccine uses the PER.C6 cell line (embryonic retinal cells), while Sputnik V is produced with the HEK293 cell line, “which has long been safely used for the production of biotechnological products.” However, the PER-C6 cell line is being used in several biologic products in development, including vaccines, gene therapies, antibodies and other therapeutic proteins.