How should vaccination against Covid-19 work?

This week there’s been a lot of excitement and some confusion about a press announcement that one vaccination against Covid-19 may become available shortly. This article looks at some of the issues as dispassionately as possible.


Vaccination isn’t really an ideal term to describe the process of being given, usually by injection, a preparation intended to induce immunity to a disease. It’s derived from a specific name, Vaccinia, for the cowpox which was originally used to confer immunity to smallpox, but is now in generic use. It’s different from immunisation, which doesn’t just refer to the administration of a vaccine, but requires that results in some degree of increased immunity. The two terms vaccination and immunisation are therefore not interchangeable, and what we’re all looking for is immunisation as a result of vaccination – that’s success.


There are currently many ways that can be used to produce a vaccine, and most of them are being tried against Covid-19. The Pfizer vaccine which has been in the news is novel because it uses a novel approach, using messenger RNA (mRNA), which is a relatively recent discovery. Although this is new in humans, the underlying science isn’t so new, and well understood. There is still much to learn about how well mRNA-based vaccines will work, but this really isn’t as unknown as some might like to make it out to be.


Much fuss has been made about the storage requirements of the new Pfizer vaccine. For many, -70˚C seems almost unattainable in ordinary life. If you’ve ever worked in a lab handling tissues and similar biological samples, you’ll probably already have come across freezers which maintain specimens at -80˚C. Even if those are in short supply, there’s one not uncommon commercial product which is invaluable here: dry ice, first sold in quantity in 1925, and widely used for storing ice cream when sold from mobile carts, and for removing warts.

Besides, the demand for vaccine is going to result in a ‘Just in Time’ supply chain, in which a fresh batch of vaccine will arrive by air and will have been injected within a couple of days. As the Pfizer vaccine can be kept safely for up to five days in a normal refrigerator, the main logistic problem isn’t going to be storage, but supply in the first place.


In common with most, if not all, the contenders, the Pfizer vaccine is given in two doses, between three and four weeks apart. This is common practice for this type of vaccine, and is normally believed to be essential in order to get a good immune response in as many people as possible. This is the procedure under which this vaccine has been trialled, and trying to cut that to a single shot would guarantee significantly lower levels of immunisation, and almost certainly make the vaccination a waste of time and effort.

The requirement for two doses doubles the logistic burden, of course. For a country of 50 million people, that means that 100 million doses of vaccine will be required (for 100% coverage, but see below), and that each person needs to attend twice, at the right time interval.

Until the second vaccination has taken effect, usually around seven days after it is given, we should behave as if there has been no immunisation. Although our immune systems should show responses soon after the first dose, these aren’t reliable or sufficient to provide any protection against Covid-19. So, when you’ve had your second shot, you can’t walk straight out and trash your mask and stop distancing: full protection will need to be maintained for at least a week, possibly longer.


As Covid-19 is generally most severe in older adults, and commonly is a very mild illness in children, most if not all countries are likely to make older adults top priority, and won’t even try vaccinating children under the age of about 12 to 15. Different explanations are being given for this, but another important factor is the fact that vaccinations intended for use in children normally require separate trialling on children of appropriate ages. We are even more careful about approving children’s vaccinations, so for the time being this isn’t deemed a priority.

How many need to be immunised?

Governments and the press have been reticent about explaining how many of the population will need to be successfully immunised before that in itself will drive down the risk of Covid-19 infection in a population.

The fundamental requirement is what’s known as the Herd Immunity Threshold (HIT), which is defined in terms of R0, the reproduction rate in a mixed, fully susceptible population. HIT is widely agreed to be estimated as
HIT = 1 – (1/R0)

There are many different estimates for the R0 of Covid-19, but most range between 2 and 3, making the HIT anything between 50% and 67%. That isn’t the proportion of the population who are vaccinated, remember, but the overall proportion who have effective immunity.

More than half the population who could catch Covid-19 need to be immune to it, so it’s not difficult to work out how many need to be vaccinated in the first place, to achieve that. In the UK, for example, around 20% of the population are children, who aren’t going to be vaccinated in the first place, leaving 80% who are available for vaccination. A significant proportion, between 5-10%, of those have already had Covid-19, although many of those will not have any lasting immunity to it as a result.

Assessing what proportion of those vaccinated achieve useful immunity against Covid-19 is more difficult, and one of the purposes of each vaccine’s trial. That’s unlikely to exceed 90%, and may be significantly less. That figure also tends to reduce over time after the second dose. Thus to achieve 67% of the population with useful immunity, for an R0 of 3, almost all (94%) of the 80% who are adults need to be vaccinated. There’s more leeway if the R0 is as low as 2, which would require around 70% of adults to be vaccinated.

In a country with a higher proportion of children, such as Kenya with over 50%, achieving the HIT without vaccinating children is impossible even with an R0 as low as 2.

There are factors which can reduce the effective numbers required to be immunised in order to limit transmission to the point where Covid-19 can be eradicated. One is, of course, that naturally acquired immunity can reduce the percentage of adults who require vaccination. This may be valuable in some smaller countries which have had very high rates of infection, but is unlikely to help much in others. It also relies on performing serological testing to determine who does have residual immunity.

Perhaps the best way to reduce the required percentage is to reduce the effective R value well below 2, using the same measures that we’ve become accustomed to, including quarantine, distancing, masks, and assiduous contact tracing. As we know, those aren’t easy and have their own impacts. But next year they may prove to be decisive in making vaccination campaigns successful. What we need next are three or more vaccines which are known to be (relatively) safe and effective, providing immunity of sufficient duration, and available in sufficient quantities to support vast vaccination campaigns.

My fingers are crossed.