Thinking of skipping your second COVID-19 vaccine shot? Experts explain what happens when you choose to skip this crucial step.

Why is a second booster dose important?

Second dose is crucial because it provides better protection and higher effectiveness. According to studies cited by CDC (Centers for Disease Control and Prevention), data shows that a single dose of Pfizer-BioNTech vaccine provides between 60-70% effectiveness. However, with the second vaccine or booster, it reaches up to 90% effectiveness.

With the Oxford-AstraZeneca vaccine, single dose efficacy is probably at 70% against symptoms for at least 90 days after first dose. Booster shots generally allow our immune system to retain memory on the virus for a longer period. The second-dose vaccine also works better against the variants. It’s also important to have the second dose to prevent the efficacy of the vaccine to dissipate into a flattened curve. The first dose allows the body to create an immune response whilst the second dose provides a booster and then strengthens the immunity to the COVID-19 virus. However, currently, the only vaccine that needs a single dose is the Johnson & Johnson vaccine.

What kind of protection does only one shot provide against COVID-19?

According to Pfizer data published in December 2020, the Pfizer-BioNTech vaccine provides approximately 52% effectiveness after the first dose. The true value lies between 29.5% and 84.5%, according to an FDA document. Of note, a single dose provides 100% protection against hospitalisation but the numbers were small.

For the Oxford-AstraZeneca vaccine, a single dose is 76% effective against COVID-19 with symptoms for at least 90 days or three months, as published in the Lancet. The study authors reported that a single dose provides 100% protection against hospitalisation but the numbers were small.

For the Sinovac vaccine, the data from Brazil did show that there was 51% efficacy for symptomatic COVID-19 infection and 100% against severe COVID-19 and 100% against hospitalisation starting 14 days or two weeks after receiving the second dose.

The first dose of the Sinovac vaccine provides very little or weak protection. In Chile, the data showed that there is only 16% efficacy after a single dose of Sinovac and 67% efficacy after the second dose. The study also found 80% effectiveness in preventing death from COVID-19 two weeks or 14 days after the second dose.

Can I receive my second dose of a COVID-19 vaccine later than usual?

Yes, you can receive your second dose of COVID-19 vaccine later than usual, but it is best to be taken at the advised time. Therefore, the clinical evidence for Pfizer-BioNTech vaccine is to take it after three weeks whilst for Oxford-AstraZeneca vaccine, the timeframe is after eight to 12 weeks as there is better efficacy after a 12-week gap.

As for the Sinovac vaccine, it is to be taken ideally after two weeks from the first dose. However, if you receive the second vaccination shot either too early or later than recommended, you do not have to restart the vaccination series.

As more data emerges, the guidelines will change with time. You have not been fully vaccinated if it has been less than two weeks since your first-dose shot; or it has been less than two weeks since your second shot; or you still need to get your second dose of your two-dose vaccination.

Can I get my second dose from another manufacturer?

Yes, it is possible to receive the second dose from a different manufacturer, but CDC said that this should only be in exceptional circumstances. An exceptional case might be one where a patient does not remember which vaccine they received and does not have documentation showing the brand and another case might be if the first-dose brand is no longer available.

Therefore, ideally, one should take the second dose from the same manufacturer.

Although Pfizer-BioNTech and Oxford-AstraZeneca vaccines use different technology, they both work to induce an immune response to the SARS-CvN-2 spike protein. All these vaccines code for the same spike protein, and there are current and ongoing clinical trials and data that can mix and match these different vaccine platforms. There are no theoretical reasons why it should not be safe to mix and match different vaccines from different manufacturers, but we have to wait for publication of the latest clinical data from the trials.

-- BERNAMA

Dr Saiful B Kassim is with Gleneagles Hospital Kuala Lumpur. His speciality is Endocrinology, Internal Medicine and Bariatric Surgery.