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Explained: The shelf life of Covid-19 vaccines

How long does a vaccine stay potent? This depends on stability tests, and can be extended based on data, as has happened with Covishield and Covaxin. What data are analysed, and how is shelf life determined?

Written by Anuradha Mascarenhas , Kaunain Sheriff M | New Delhi, Pune |
Updated: January 6, 2022 1:53:56 pm
Teenagers queue up for their doses in New Delhi on Monday, as India began vaccination for the 15-18 age group. (Photo: Amit Mehra)

More than 40 lakh teenagers in the 15-18 age group received their first dose of Covid-19 vaccines on Monday as India began its drive to vaccinate the younger population groups. Some concerns were raised about ‘expired’ Covaxin being administered to this younger group, leading to the Health Ministry issuing a clarification.

The Ministry described these claims as “false and misleading” and said these were based on “incomplete information”. It pointed out that the shelf life of Covaxin, the only vaccine being given to the people below 18 years of age, had been extended in November after proper regulatory scrutiny, and as such these vaccine doses were as good as any.

What led to the concerns?

The anxiety arose after several people pointed out that vaccine batches that were supposed to expire in November were being administered to the younger people on Monday. The government later clarified that the shelf life of these vaccines had been extended from nine months to 12 months in November itself, and therefore there was nothing wrong with these batches.

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The Health Ministry said the shelf life of vaccines is extended by the national regulator, Central Drugs Standard Control Organisation, or CDSCO, based on a comprehensive analysis and examination of stability study data furnished by the vaccine manufacturers. It also pointed out that CDSCO has earlier approved the extension of shelf life of Covishield as well, and nothing special had been done to Covaxin.

Why was the date of expiry of Covaxin extended?

In response to an application from Bharat Biotech, the manufacturer of Covaxin, the CDSCO on October 25, 2021 approved the extension of the shelf life of this indigenously developed vaccine from 9 to 12 months from the date of manufacture. This approval was based on the availability of additional “stability data” which was submitted by the firm to CDSCO.

With the shelf-life extension, hospitals could utilise the stock that was nearing expiry and avoid vaccine wastage. An estimated 20 crore vaccine doses are required to fully vaccinate the nearly 10 crore people in the 15-18 age group.

In a recent statement, Bharat Biotech had said that they had documented excellent safety and immunogenicity data readouts in children. More than 15 crore Covaxin doses have been administered in the country to adults since vaccination commenced last year.

What are ‘stability’ and ‘shelf life’ of a vaccine?

Vaccines are complex mixtures of proteins, carbohydrates, lipids, inactivated virus, or adjuvants —which are substances that are intended to enhance immune response and subsequent clinical efficacy of the vaccine. These together contribute to overall vaccine efficacy and safety.

Like other medicinal products, vaccines come with a date of expiry and shelf life determined by the manufacturer and approved by regulatory authorities. The constituents of a vaccine can go bad over time because of slow chemical reactions and lose efficacy.

According to the World Health Organization (WHO), stability is the ability of a vaccine to retain its chemical, physical, microbiological and biological properties within specified limits throughout its shelf life.

A series of tests are designed to obtain information on the stability of a vaccine in order to define its shelf life and utilisation period under specified packaging and storage conditions. And depending on the nature of the antigen and other components, and the manufacturing process, stability parameters are selected on a case-by-case basis, the WHO guidelines state.

There are three specific objectives of stability studies, which differ throughout a vaccine’s lifetime. First, it is conducted to determine shelf life and storage conditions. Second, the stability studies, monitor vaccine stability in the post-licensure period, that is, when the vaccine is marketed commercially. Third, according to the WHO guidelines, stability studies are conducted to support manufacturing changes by demonstrating comparability of product manufactured by different processes.

How is shelf life calculated?

Shelf life is calculated by storing the product at different temperatures for various lengths of time and then testing its effectiveness, leading virologist Dr Shahid Jameel said. By storing the particular product at various temperatures, and then periodically checking if there is any degradation of the product, an expiry date is arrived at.

The duration over which the product is stable and effective in the specified conditions is considered its shelf life. There are various biochemical ways in which the degradation can be estimated, said Dr Vineeta Bal, leading immunologist.

According to the WHO guidelines, the shelf life of a vaccine is the period of time during which the vaccine, if stored correctly, is expected to comply with the specification, as determined by stability studies on a number of batches of the product. The shelf life is used to establish the expiry date of each batch.

For vaccines, it is done by injecting into small animals (typically mice) to assess if the ability to make antibodies goes down with storage time and temperature, Dr Jameel said. An expiry date means that roughly beyond that, the vaccine won’t raise immunity as well as earlier. It may still work but sub-optimally, he said.

“The shelf life of a vaccine is a reflection of how long the vaccine retains its potency and stability at a given storage temperature and therefore its effectiveness. The shelf life is used to establish the expiry date of each batch of the vaccine product. Expiry dates do not affect the safety of the vaccine, rather are related to the potency or amount of protection the vaccine gives,” the WHO regional office for Africa had said in a statement in May last year.

The WHO had said that any extension in the shelf life will only apply to vaccines not yet labelled and distributed. Hence the expired or near-to-expire doses in distribution for use will not be affected by any future decision of shelf life extension.

Is this extension of date a special favour given to Covaxin?

No. As mentioned, the CDSCO had in February last year extended the shelf life of Covishield from nine months to 12 months, after a similar process. The manufacture of the vaccine has to submit data to show that its vaccine retains its stability for longer periods. If the regulator is satisfied with the data, it can extend the expiry period.

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