Sample Size for COVID-19 Vaccine Phase 3 Study

Vaccine development for COVID-19 is at historic speed. Several companies have already had vaccines in the early phase of clinical trials. Leading the pack are AstraZeneca / Oxford University and Moderna / NIH who are targeting July for starting the late-stage phase 3 studies. Phase 3 study is pivotal in demonstrating that the vaccine is safe and effective in the general population including those at high risk of contracting COVID-19 and those with underly diseases.

There are quite some discussions about the sample size of the phase 3 study - the sample size for phase 3 study should be adequate to provide sufficient statistical power to demonstrate the vaccine's efficacy in preventing the COVID-19 infection and also adequate for regulatory agencies to assess the safety of the vaccine - sample size should be big enough so that the rare event (if any) can be observed. The sample size is being proposed to be at least 30,000 volunteers.

• FDA Shouldn't Authorize Coronavirus Vaccines Unless They're Tested on at Least 30,000 People: Experts
• Vaccine experts want the FDA to commit to 30,000 people in Covid-19 vaccine trials
• Lawmakers ask FDA to outline COVID-19 vaccine approval process
• Opinion: Here’s why the FDA may approve a Covid-19 vaccine before the November elections, according to Jefferies’ biotech-research team

For pivotal clinical trials, the sample sizes depend on the study design and the primary efficacy endpoint; and estimated based on assumptions.

The study design will be traditionally randomized, double-blinded, placebo- or active-controlled, parallel-group design (not human challenge design even though it has been pushed by some people) - see a previous post "Human Challenge Study Design for Covid-19 Vaccine Clinical Trials?".

• Human challenge trials with live coronavirus aren’t the answer to a Covid-19 vaccine
• Human Challenge Study Design for Covid-19 Vaccine Clinical Trials?

The primary efficacy endpoint is the incidence of symptomatic COVID-19. Moderna has finalized the study design for its phase 3 study and it says that the study will include 30,000 volunteers and the primary and secondary efficacy endpoints as the following:

"The primary objective of the trial, which is set to start in July, is to assess the ability of mRNA-1273 to prevent symptomatic COVID-19 disease. Secondary endpoints will assess the ability of mRNA-1273 to prevent hospitalization and infection with SARS-CoV-2."

•  Fauci: Moderna’s Phase 3 Covid-19 Vaccine Trial Of 30,000 Individuals Will Begin In July
• Moderna finalizes design of phase 3 COVID-19 vaccine trial ahead of July start
• Exclusive: U.S. plans massive coronavirus vaccine testing effort to meet year-end deadline

AstraZeneca/Oxford's COVID-19 vaccine enters phase 2/3 clinical trial. Their proposed sample size is much smaller than 30,000 subjects mentioned by US experts. We can also notice that they propose to use the vaccine against meningococcal bacteria as the control (instead of placebo). 

"Researchers at the University of Oxford have begun enrolling subjects in a phase 2/3 clinical trial of AstraZeneca-partnered COVID-19 vaccine AZD1222. The next stage of the program, which follows a 1,000-subject phase 1, is set to enroll 10,260 people in the U.K. to generate results to support the first shipments to customers in September."

"Once the vaccine moves into phase 3, the researchers will limit enrollment to people age 18 years and older. Adult participants in the phase 2 and 3 trials will be randomized to receive one or two doses of AZD1222 or a vaccine against meningococcal bacteria that will serve as the control.

The use of an active vaccine as a control is intended to ensure participants are unable to tell whether they received AZD1222 based on side effects such as soreness at the injection site. In the absence of such effects across both groups, participants could determine whether they had received the vaccine and make behavioral changes that skew the results of the study. "

Baseline on the study registration on clinicaltrials.gov "A Phase 2/3 Study to Determine the Efficacy, Safety and Immunogenicity of the Candidate Coronavirus Disease (COVID-19) Vaccine ChAdOx1 nCoV-19", we can see the following:

• The study consisted of different sub-groups (different age groups and different duration of the follow-up period).
• The primary efficacy endpoint is the number of biologically confirmed (PCR positive) symptomatic cases of COVID-19
• The primary safety endpoint is the occurrence of serious adverse events (SAEs) throughout the study duration.

It is not clear what the assumptions are used for estimating the sample size (30,000 subjects for Moderna's phase 3 study or 10,260 subjects for AstraZeneca/Oxford's phase 3 study). Perhaps the sample size is more based on the experiences rather than the calculation based on the solid assumptions.
In order to estimate the sample size, we will need to know the incidence rate of symptomatic COVID-19 in the control group (subjects who receive a placebo or a none anti-COVID-19 vaccine) and then the effect size (how much reduction in the incidence rate of symptomatic COVID-19 in the vaccine group (subjects who receive COVID-19 vaccination) - both of these are difficult to obtain.

• Estimating COVID-19 Prevalence in Symptomatic Americans
• COVID-19 Cases in U.S.

The incidence rate of symptomatic COVID-19 can vary significantly depending on the timing, the location, and the prevention and COVID-19 control strategies (i.e., stay-at-home, quarantine, social distance).

Back in January and February, the incidence rate of symptomatic COVID-19 would be very high. But now the COVID-19 situation in China has been under control. It is no longer feasible to test the efficacy of the COVID-19 vaccine in China. There are reports that COVID-19 vaccine researchers in China are looking for foreign sites to recruit the volunteers to test the efficacy of their vaccine candidate.

In the US, the incidence rate of symptomatic COVID-19 would be very high in New York and New Jersey in March/April time, now the states with high incidence rates have shifted to the southern states such as Texas, Arizona, North Carolina.

For phase 3 study to demonstrate the efficacy against the COVID-19, a sufficient number of subjects need to be included in the study so that (hopefully) a sufficient number of symptomatic COVID-19 cases can be observed (more in the control group and less in COVID-19 vaccine group). If subjects are recruited in areas with a high incidence rate, it will be quicker to accrue the number of symptomatic COVID-19 cases. Ironically, with strict COVID-19 prevention/control strategies, by the time the phase 3 studies start, the situation may be under the control and the incidence rate may be too low to accrue enough symptomatic COVID-19 cases. In the article below, a drop in coronavirus cases was listed as one of the biggest risks for Moderna's phase 3 study.

Here Are the 3 Biggest Risks for Moderna Right Now

"3. A drop in coronavirus cases

Of course, a drop in coronavirus cases is great news for everyone. But in order for Moderna and other vaccine makers to test their investigational products, the virus must be actively circulating. In an outbreak situation, the vaccine or placebo is administered to a group of healthy volunteers. If a high number of placebo participants get sick and those who received the vaccine don't, it's likely the vaccine is working. But if the virus is hardly present, it would be impossible to draw such a conclusion.

In the U.S., coronavirus cases recorded by the Centers for Disease Control and Prevention are cumulative, so they will continue to grow. And some states are still seeing spikes. But nationally, the number of people seeking medical attention for symptoms has been on the decline. If the trend continues, it may present difficulties for Moderna and rivals conducting trials in the U.S. to prove vaccine efficacy.

That doesn't mean the vaccine is doomed, but testing it could become more complicated. If virus circulation drops considerably, researchers may conduct a "challenge" trial. That means healthy volunteers are immunized, then exposed to the virus. There also is the possibility of conducting trials in other areas where the virus is on the rise. At this point, Moderna hasn't said it would turn to either of these alternatives."

The primary efficacy endpoint of "biologically confirmed symptomatic cases of COVID-19" belongs to the count data. Given the incidence rate is very low among the general population, the data can be assumed to follow a Poisson distribution or negative binomial distribution. The sample size calculation will then need to be based on Poisson rates or negative binomial rates. 

Hypothetically, the statements about the sample size for phase 3 COVID-19 vaccine studies can be something like this assuming 50% reduction in symptomatic CIVID-19 cases in the vaccine group:

Assuming that the incidence rate (Poisson mean) of subjects with symptomatic COVID-19 case is 0.003 (0.3%) in control group and 0.0015 (0.15%) in COVID-19 vaccination group, 32,673 subjects needs to be randomized to have 80% statistical power to detect the treatment difference."

In the US, we saw the rise in coronavirus cases - it is bad, really bad, but ironically it is good for vaccine clinical trials - it is easier and quicker to accrue a sufficient number of symptomatic COVID-19 cases and it requires less sample size for phase 3 studies to demonstrate the difference between vaccinated and control groups. As Dr. John Skerritt from the Department of Health, Australia said about the COVID-19 pandemic: "Do not waste a good crisis". 

Neutralizing Antibodies: Active Immunization and Passive Immunization Against Covid-19

Generally speaking, a person achieves immunity to disease through the presence of neutralizing antibodies, or proteins produced by the body that can neutralize or even destroy toxins or other disease carriers.  Active immunization is the process of vaccination to prevent an infectious disease by activating the body’s production of antibodies that can fight off invading bacteria or viruses.

The neutralizing antibodies (so-called because they stop the virus from being able to infect cells) can also be obtained from outside the body and can be given the recipients as a therapy for the prevention or treatment of a disease. Passive immunization is the process of administering the antibodies against a particular infectious agent.

An easy example is the antibodies against the rabies. In order to get immunity against rabies virus, we can receive rabies vaccine. Usually 2-3 weeks after the administration of the rabies vaccine (usually several courses), the neutralizing antibodies against rabies will develop in recipient’s body – this is called vaccine-induced active immunity. However, rabies antibodies can also be obtained from the human plasma donated by people in plasma collection centers scattered throughout the United States. The pooled plasma can be fractionated, and rabies antibodies can be obtained – these products are called RIG (Rabies Immune Globulin or hyperimmune globulin against rabies). The RIG can be given to the people to obtain so called ‘passive immunity’. If someone had potential exposure to rabies (for example, bite by wild animals) and had no record of rabies vaccination, the RIG should be immediately given to achieve the passive immunity for short period protection.

The same process of active immunity and passive immunity applies to the Covid-19 situation. Below is a table to compare the active immunity vs. passive immunity in Covid-19 situation:

Active Immunity	Passive Immunity
Relies on neutralizing antibodies against SARS-CoV-2 (the virus causing Covid-19)
active immunity results when exposure to a disease organism triggers the immune system to produce antibodies to SARS-CoV-2	passive immunity results when receiving a therapy containing neutralizing antibodies against SARS-CoV-2
Neutralizing antibodies are generated by our own immune system	Neutralizing antibodies are manufactured or obtained outside the body and then given to the recipients
Active Immunity can be obtained in two ways: Natural Immunity: obtaining immunity because of infection with Covid-19 (whether it is symptomatic or asymptomatic) Vaccine-Induced Immunity: obtaining immunity by receiving the vaccine (vaccination) that won’t make someone sick, but will trigger the body to make neutralizing antibodies), which is known as vaccine-induced immunity	passive immunity is provided when a person is given neutralizing antibodies. Antibody-containing blood products: convalescent plasma obtained from Covid-19 recovered patients Hyperimmune products (containing concentrated neutralizing antibodies) Manufactured antibody products
It will take a while for the body to generate neutralizing antibodies	Off-the-shelf, ready to use
Immunity (once obtained) will be longer	Immunity will be shorter
Companies who are working on vaccines: see previous post “Coronavirus Vaccine Tracker - Developing Vaccines Against Covid-19 “	Key players in the field (examples only): Convalescent plasma – Mayo Clinic Hyperimmune globulin – Grifols Neutralizing antibodies cocktail - Regeneron, Eli Lilly, Sorrento Therapeutics
Clinical trials in ‘healthy’ volunteers	Clinical trials in Covid-19 patients
Clinical trials to demonstrate the effect in prevention (prevent from symptomatic Covid-19)	Clinical trials to demonstrate the effect in treatment (speed up the recovery of the symptomatic Covid-19 patients and decrease the mortality)
The development process is longer	The development process is shorter
Larger sample size for clinical trials required for demonstrating the efficacy in prevention of Covid-19	Smaller sample size for clinical trials required for demonstrating the efficacy in the treatment of Covid-19

References:

• What to Know About Active Vs. Passive Immunity—And Why Both Matter With COVID-19
• Explainer: What are antibody therapies and who is developing them for COVID-19?
• A coronavirus vaccine is still months away, but an antibody treatment could be closer
• Coronavirus Antibody Therapies Raise Hopes—and Skepticism
• "Immunity passports" in the context of COVID-19
• CDC: Immunity Types
• CDC: Rabies 
• China begins phase 1 clinical trials of neutralizing antibody treatment for COVID-19

Coronavirus Vaccine Tracker - Developing Vaccines Against Covid-19

Ultimately, to win the battle against the Covid-19, we will rely on a safe and effective available in very large quantities. Pharmaceutical companies (large or small), biotech companies, academic, and governmental agencies have all rushed into the vaccine development field to fight the Covid-19. We hope that some vaccines will eventually become the winners.

The New York Times has a website for "Coronavirus Vaccine Tracker". The tracker grouped the vaccines into four categories:

• Genetic Vaccines developed by Moderna, BioNTech/Pfizer, Inovio, ...
• Viral Vector Vaccines developed by Oxford/Astrazeneca, CanSinoBIO, Johnson and Johnson,...
• Protein-Based Vaccines developed by Novavax, GSK, Baylor College of Medicine,...
• Whole-Virus Vaccines developed by Sinovac, Sinopharm,...

So far, some vaccines have gone beyond the pre-clinical stage and progressed into the clinical trial stage. Below is a timeline for those vaccines already in human trials: 

Also, this map shows where coronavirus vaccines are being tested around the world.

However, vaccine development is a lengthy process consisting of phase 1 -> phase 2 -> phase 3 clinical trials before the vaccine can be approved for use. In phase 3 study, the candidate vaccine must be demonstrated to be safe in the general public (with different age groups including the population with underly conditions)

With so many clinical trials initiated or to be indicated, there may be a shortage of volunteers for these studies. The extreme measures to control the Covid-19 (guarantee, stay-at-home, face-masks,...) are necessary to prevent the virus spread, however, it will lower the incidence rate of Covid-19 - consequently, the phase 3 studies for vaccines will require larger sample size with more participants in order to have the adequate statistical power and accumulate a sufficient number of Covid-19 infection cases to establish the efficacy of the candidate vaccine. The phase 2 study will include around 1000 volunteers and the phase 3 study will be even bigger with tens of thousands of volunteers. For those vaccines already in the clinical trial stage: the University of Oxford's phase 1/2 trial had a sample size of 1,090 participants. Moderna's vaccine candidate is being tested in phase 2 study with 600 participants. Its phase 3 study is planned to start this July and will include 30,000 participants to test if the vaccine is safe in general population and if the vaccine is effective in preventing symptomatic Covid-19.


Can we really expect to have a safe and effective Covid-19 vaccine available in a year? It is unlikely, but some people are cautiously optimistic. Read the panel discussions below:

Can a Vaccine for Covid-19 Be Developed in Record Time?

A discussion moderated by Siddhartha Mukherjee

Should Drug Safety (or Pharmacovigilence) Group be Provided with the Randomization Schedule for Blinded Studies?

To avoid the conscious and unconscious bias in safety and efficacy assessment, clinical trials are usually designed as double-blinded studies whenever the blinding is feasible. Double-blind indicates that the treatment assignments are concealed to the investigator and the study participants. The blinding is usually extended to the entire study team including the sponsor, CRO, and vendors. If too many study participants are unblinded during the study, the study integrity will be compromised.

However, not everybody is blinded to the treatment assignment. On the Sponsor side, the clinical trial material (CTM) management group is usually unblinded and has full access to the treatment assignments - because they need to make sure that the correct drugs (active or control) are packaged, labeled, shipped, and dispensed. CTM group will track the inventory at the study sites to make sure the study materials are available at all open sites.

The drug safety group or pharmacovigilance group (PVG) may also request for full access to the treatment assignment and they claim the full access to the treatment assignment is needed for serious adverse event reporting.

It is true that the regulations require the drug safety group to report the 'serious and unexpected suspected adverse reaction (SUSAR)' to regulatory agencies, investigators, and IRBs/ECs.

• 21CFR 312.32 IND safety reporting
• Directive 2001/20/EC of the European Parliament and of the Council
• Breaking the Blind in Clinical Trials & Reporting to Health Authorities, Investigators & IRBs/Ethics Committees
• Clinical Safety Data Management: Definitions and Standards for Expedited Reporting
• Safety assessment in Clinical Trials and Beyond

Code of Federal Regulation regarding IND Safety Reporting says:

"(i) Serious and unexpected suspected adverse reaction. The sponsor must report any suspected adverse reaction that is both serious and unexpected. The sponsor must report an adverse event as a suspected adverse reaction only if there is evidence to suggest a causal relationship between the drug and the adverse event, such as:

(A) A single occurrence of an event that is uncommon and known to be strongly associated with drug exposure (e.g., angioedema, hepatic injury, Stevens-Johnson Syndrome);

(B) One or more occurrences of an event that is not commonly associated with drug exposure, but is otherwise uncommon in the population exposed to the drug (e.g., tendon rupture);

(C) An aggregate analysis of specific events observed in a clinical trial (such as known consequences of the underlying disease or condition under investigation or other events that commonly occur in the study population independent of drug therapy) that indicates those events occur more frequently in the drug treatment group than in a concurrent or historical control group."

EU has similar regulations. According to EU directive "on the approximation of the laws, regulations and administrative provisions of the Member States relating to the implementation of good clinical practice in the conduct of clinical trials on medicinal products for human use"

Article 17 Notification of serious adverse reactions 1. (a) The sponsor shall ensure that all relevant information about suspected serious unexpected adverse reactions that are fatal or life-threatening is recorded and reported as soon as possible to the competent authorities in all the Member States concerned, and to the Ethics Committee, and in any case no later than seven days after knowledge by the sponsor of such a case, and that relevant follow-up information is subsequently communicated within an additional eight days. (b) All other suspected serious unexpected adverse reactions shall be reported to the competent authorities concerned and to the Ethics Committee concerned as soon as possible but within a maximum of fifteen days of first knowledge by the sponsor. (c) Each Member State shall ensure that all suspected unexpected serious adverse reactions to an investigational medicinal product which are brought to its attention are recorded. (d) The sponsor shall also inform all investigators.

It is true that the drug safety group needs the treatment assignment information to assess the causal relationship of a SUSAR event to the study drug. If a SUSAR occurs in a placebo group, the event will then not be required to be reported.

Unlike the STM group, the drug safety group only needs the treatment assignment for individual subjects at the time when a SUSAR is reported. It is concerning that the drug safety group is given full access to the treatment assignment for all subjects. Drug safety group personnel sometimes can accidentally communicate the treatment assignment information outside the drug safety group (to the investigator and to the blinded study team) and causing the accidental unblinding of the subject.

I just read the FDA’s review document for a Merck product and noticed the review comments mentioning that drug safety staff should be blinded during the study. Given the potential accidental unblinding caused by drug safety staff, it is prudent to restrict drug safety staff’s access to the randomization codes for ongoing blinded studies.  

https://www.accessdata.fda.gov/drugsatfda_docs/nda/2014/204886Orig1s000MedR.pdf

However, the drug safety group should be given emergency access to the randomization codes. In the situation a SUSAR is reported, drug safety personnel who are handling the SUSAR reporting can log into the interactive response technology (IRT) including interactive web response (IWR) and interactive voice response system (IVR) systems, and obtain the randomization information and treatment assignment for the individual subject with SUSAR reported. 

In summary, the drug safety group or PVG should not be given full access to all randomization codes, but should be provided in a controlled way the access to treatment information for individual subjects who have SUSAR reported.